diff --git a/BUILDING.md b/BUILDING.md
new file mode 100644
index 0000000..3999851
--- /dev/null
+++ b/BUILDING.md
@@ -0,0 +1,110 @@
+# Linux build
+
+The project targets **Unity 2018.1.6f1**. Keep this version for now: the
+simulation uses the bundled UNet networking API and the legacy .NET runtime.
+No engine migration has been made.
+
+## Toolchain
+
+The following tools were installed and launched on this Debian 13 x86_64 host:
+
+- Unity Editor: `/opt/unity/2018.1.6f1/Editor/Unity`, including Linux player support.
+- Blender 2.79b: `/opt/blender-2.79b-linux-glibc219-x86_64/blender`, also available
+  as `blender` on PATH. The project's Blender models use the 2.79 file format.
+- GTK 2, libsoup 2.4, GLU, and GConf 2 compatibility libraries. GConf was installed
+  from Debian's `libgconf-2-4_3.2.6-8_amd64.deb` and
+  `gconf2-common_3.2.6-8_all.deb`; Debian 13 no longer ships those packages.
+- Xvfb and xauth for running the old editor without a desktop display.
+
+Downloads: [Unity 2018.1.6f1](https://unity.com/releases/editor/whats-new/2018.1.6f1),
+[Blender 2.79b](https://download.blender.org/release/Blender2.79/),
+[Debian GConf packages](https://deb.debian.org/debian/pool/main/g/gconf/).
+
+## Build command
+
+Once Unity is activated and the damaged source assets below are restored:
+
+```sh
+./build-linux.sh
+```
+
+For an editor installed elsewhere:
+
+```sh
+UNITY_EDITOR=/path/to/2018.1.6f1/Editor/Unity ./build-linux.sh
+```
+
+The intended output is `Builds/linux/Simulation.x86_64`, with its accompanying
+data directory. The editor log is `Builds/linux/build.log`. The build uses
+`Assets/Scenes/Main.unity`; the original settings incorrectly named the absent
+`SampleScene.unity` despite already containing Main's GUID.
+
+The existing standalone startup code selects the client menu. This build command
+preserves that behavior.
+
+## Current blockers and verification
+
+**No Linux executable has been built.** The editor starts, but the attempted
+batch build exits with status 1 at license activation:
+
+```text
+Failed to activate/update license. Missing or bad username and password.
+```
+
+Unity Personal is free for eligible users, but still requires activation.
+For a desktop session, sign in to Unity Hub, then use Settings > Licenses >
+Add license > Get a free personal license if it is not activated automatically.
+See [Unity's Hub license instructions](https://docs.unity.com/en-us/hub/manage-license).
+
+For SSH access, Unity CLI 1.0.0-beta.9 is installed at
+`/root/.local/bin/unity`, along with GNOME Keyring and `libsecret-tools`.
+Its login command prints a URL that can be opened in a browser on another
+device. Run `./unity-ssh.sh` as the same Linux user that will run the build.
+It starts a private D-Bus session and a dedicated Unity keyring under
+`~/.local/share/unity-ssh/keyrings`, leaving the desktop login keyring intact.
+Choose a password on first use and use the same password when reopening the
+helper. This password protects saved credentials locally; enter your Unity
+account password only on Unity's sign-in page. The helper verifies that the
+keyring can store and retrieve a test credential before opening a shell.
+In that shell, run:
+
+```sh
+unity auth login
+unity license activate --personal --accept-eula
+unity license status
+```
+
+The activation command accepts the Unity Personal license terms. Type `exit`
+when finished to close the shell and its keyring session.
+
+The command options and browser-assisted login startup have been checked here.
+Keyring tests with temporary credentials cover creation, reopening, incorrect
+password rejection, inherited desktop sessions, and concurrent-session
+rejection. The CLI's own credential-store check resolves the Secret Service
+backend inside the helper. Run `./unity-ssh.sh --check` to check the keyring
+without starting a Unity sign-in.
+Sign-in, activation, and compatibility with the old editor still need to be
+verified. See [Unity CLI instructions](https://docs.unity.com/en-us/unity-cli/use-unity-cli)
+and [release notes](https://docs.unity.com/en-us/unity-cli/release-notes).
+
+There is also existing file corruption, independent of activation. An audit
+found **33 asset files whose contents are Unity metadata instead of their
+expected source/model/image/material data**. Five of these are C# source files:
+
+- `UnityProject/Assets/Scripts/CarLL.cs`
+- `UnityProject/Assets/Scripts/misc/trafficLightColor.cs`
+- `UnityProject/Assets/Standard Assets/CrossPlatformInput/Scripts/VirtualInput.cs`
+- `UnityProject/Assets/!ext/PostProcessing/Editor/PostProcessingInspector.cs`
+- `UnityProject/Assets/!ext/PostProcessing/Editor/Monitors/WaveformMonitor.cs`
+
+A separate compilation attempt with Unity's bundled Mono compiler and genuine
+2018.1 assemblies fails with `CS8025: Parsing error` in `VirtualInput.cs`.
+The full asset list is in the local `Builds/linux/source-audit.txt`; compiler
+output is in `Builds/linux/script-check/compile.log`.
+
+Some metadata is damaged too: there are duplicated GUIDs, source code stored in
+`.cs.meta` files, and empty directories with `.meta` names. The file at
+`UnityProject/Scripts/CarLL.cs` is not a usable backup: it contains a
+`TrafficController` class. A clean original checkout/archive is needed to
+restore these files reliably. Existing assets and the old Library cache were
+left intact.
diff --git a/UnityProject/ProjectSettings/EditorBuildSettings.asset b/UnityProject/ProjectSettings/EditorBuildSettings.asset
index 63ff1f9..57ae1c2 100644
--- a/UnityProject/ProjectSettings/EditorBuildSettings.asset
+++ b/UnityProject/ProjectSettings/EditorBuildSettings.asset
@@ -6,6 +6,6 @@ EditorBuildSettings:
   serializedVersion: 2
   m_Scenes:
   - enabled: 1
-    path: Assets/Scenes/SampleScene.unity
+    path: Assets/Scenes/Main.unity
     guid: 99c9720ab356a0642a771bea13969a05
   m_configObjects: {}
diff --git a/build-linux.sh b/build-linux.sh
new file mode 100755
index 0000000..d6c252f
--- /dev/null
+++ b/build-linux.sh
@@ -0,0 +1,42 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+repo_dir="$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" && pwd)"
+unity_editor="${UNITY_EDITOR:-/opt/unity/2018.1.6f1/Editor/Unity}"
+output_dir="$repo_dir/Builds/linux"
+log_file="$output_dir/build.log"
+
+if [[ ! -x "$unity_editor" ]]; then
+    echo "Set UNITY_EDITOR to the Unity 2018.1.6f1 Editor/Unity executable." >&2
+    exit 1
+fi
+
+mkdir -p "$output_dir"
+echo "Building Linux x86_64 player; log: $log_file"
+
+# Unity 2018 on Linux still needs an X server in batch mode.
+runner=()
+if [[ -z "${DISPLAY:-}" ]]; then
+    if ! command -v xvfb-run >/dev/null; then
+        echo "Install xvfb and xauth for builds without a display." >&2
+        exit 1
+    fi
+    runner=(xvfb-run -a)
+fi
+
+if "${runner[@]}" "$unity_editor" \
+    -batchmode -nographics -quit \
+    -projectPath "$repo_dir/UnityProject" \
+    -buildTarget Linux64 \
+    -buildLinux64Player "$output_dir/Simulation.x86_64" \
+    -logFile "$log_file"; then
+    if [[ ! -x "$output_dir/Simulation.x86_64" ]]; then
+        echo "Unity exited without producing a Linux player. See $log_file." >&2
+        exit 1
+    fi
+    echo "Built $output_dir/Simulation.x86_64"
+else
+    result=$?
+    [[ ! -f "$log_file" ]] || tail -n 60 "$log_file" >&2
+    exit "$result"
+fi
diff --git a/unity-ssh.sh b/unity-ssh.sh
new file mode 100755
index 0000000..9804f8a
--- /dev/null
+++ b/unity-ssh.sh
@@ -0,0 +1,107 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+unity_cli="${UNITY_CLI:-$HOME/.local/bin/unity}"
+if [[ ! -x "$unity_cli" ]]; then
+    echo "Set UNITY_CLI to the installed Unity CLI executable." >&2
+    exit 1
+fi
+
+# Always use a private bus, including when SSH inherited a desktop bus.
+if [[ "${1:-}" != --keyring-session ]]; then
+    exec dbus-run-session -- bash "$0" --keyring-session "$@"
+fi
+shift
+unset GNOME_KEYRING_CONTROL
+
+# Give Unity its own credential collection. Do not read or modify the user's
+# desktop login.keyring, which may be locked, damaged, or in use elsewhere.
+keyring_data_dir="${UNITY_SSH_KEYRING_DIR:-${XDG_DATA_HOME:-$HOME/.local/share}/unity-ssh}"
+umask 077
+mkdir -p "$keyring_data_dir"
+exec 9>"$keyring_data_dir/session.lock"
+if ! flock -n 9; then
+    echo "A Unity SSH keyring session is already open. Exit it before starting another." >&2
+    exit 1
+fi
+
+read -r -s -p "Unity keyring password (choose one on first use): " unity_keyring_password
+echo
+if [[ -z "$unity_keyring_password" ]]; then
+    echo "A nonempty password is required for the encrypted keyring." >&2
+    exit 1
+fi
+runtime_dir="$(mktemp -d "${TMPDIR:-/tmp}/unity-keyring.XXXXXX")"
+daemon_pid=""
+cleanup() {
+    if [[ -n "$daemon_pid" ]]; then
+        kill "$daemon_pid" 2>/dev/null || true
+        wait "$daemon_pid" 2>/dev/null || true
+    fi
+    rm -rf -- "$runtime_dir"
+}
+trap cleanup EXIT
+trap 'exit 130' INT
+trap 'exit 143' TERM
+
+# Keep the daemon on this bus, wait for it to register, and stop it on exit.
+# Only the daemon sees the alternate data directory; Unity keeps its normal
+# configuration and license-file locations.
+XDG_DATA_HOME="$keyring_data_dir" gnome-keyring-daemon \
+    --foreground --unlock --components=secrets --control-directory="$runtime_dir" \
+    < <(printf '%s' "$unity_keyring_password") >"$runtime_dir/daemon.log" 2>&1 &
+daemon_pid=$!
+unset unity_keyring_password
+
+owner='(false,)'
+for (( attempt=0; attempt<100; attempt++ )); do
+    owner="$(gdbus call --session --dest org.freedesktop.DBus \
+        --object-path /org/freedesktop/DBus \
+        --method org.freedesktop.DBus.NameHasOwner org.freedesktop.secrets)"
+    [[ "$owner" != '(true,)' ]] || break
+    kill -0 "$daemon_pid" 2>/dev/null || break
+    sleep 0.1
+done
+if [[ "$owner" != '(true,)' ]]; then
+    echo "The Unity keyring service did not start." >&2
+    cat "$runtime_dir/daemon.log" >&2
+    exit 1
+fi
+
+locked="$(gdbus call --session --dest org.freedesktop.secrets \
+    --object-path /org/freedesktop/secrets/collection/login \
+    --method org.freedesktop.DBus.Properties.Get org.freedesktop.Secret.Collection Locked 2>/dev/null)" || locked='missing'
+if [[ "$locked" != '(<false>,)' ]]; then
+    echo "Could not unlock the Unity keyring at $keyring_data_dir/keyrings." >&2
+    echo "Use the password chosen for this helper. No credential files were removed." >&2
+    exit 1
+fi
+
+# Check persistence before asking the user to complete a browser sign-in.
+probe="unity-ssh-$$-$RANDOM"
+printf '%s' "$probe" | timeout 10 secret-tool store --label='Unity SSH keyring check' unity-ssh-check "$probe"
+saved="$(timeout 10 secret-tool lookup unity-ssh-check "$probe")"
+timeout 10 secret-tool clear unity-ssh-check "$probe"
+if [[ "$saved" != "$probe" ]]; then
+    echo "The Unity keyring could not save and retrieve a test credential." >&2
+    exit 1
+fi
+echo "Unity keyring storage check passed."
+
+export PATH="$(dirname -- "$unity_cli"):$PATH"
+export UNITY_NO_CLI_INVOKED_TELEMETRY=1
+if [[ "${1:-}" == --check ]]; then
+    exit 0
+fi
+if (( $# )); then
+    "$unity_cli" "$@"
+    exit $?
+fi
+
+echo "Unity login shell ready. Run: unity auth login"
+echo "Open the printed sign-in URL in your own computer's browser."
+echo "After signing in and reviewing the Personal terms, run:"
+echo "  unity license activate --personal --accept-eula"
+echo "  unity license status"
+echo "Type exit to close this shell and its keyring session."
+bash --noprofile --norc -i