docs(l4d2): document host prerequisites

components/l4d2-host-lib/README.md

@@ -23,6 +23,51 @@ The host library uses hard-coded runtime paths under `/opt/l4d2`:
 - `/opt/l4d2/instances/<name>`
 - `/opt/l4d2/runtime/<name>/{upper,work,merged}`
 
+## Host Prerequisites
+
+The host library intentionally does not install or preflight runtime dependencies. The target host must provide them before running `l4d2ctl`.
+
+Validated on Debian 13 during the `ckn@10.0.4.128` smoke test:
+
+- Python 3.12+ with virtualenv/pip tooling for installing `l4d2host`.
+- `steamcmd` available on `PATH` and able to self-update as the runtime user.
+- 32-bit compatibility libraries for SteamCMD on amd64 Debian: `libc6-i386`, `lib32gcc-s1`, `lib32stdc++6`.
+- `fuse-overlayfs` and `fusermount3` for per-instance overlay mounts.
+- `systemctl --user` and `journalctl --user` available for the runtime user.
+- User lingering enabled when services must survive SSH sessions: `sudo loginctl enable-linger <user>`.
+- `/opt/l4d2` created and writable by the runtime user.
+
+Example Debian setup:
+
+```bash
+sudo apt-get update
+sudo apt-get install -y \
+  python3 python3-venv python3-pip \
+  curl ca-certificates tar gzip \
+  fuse-overlayfs fuse3 \
+  libc6-i386 lib32gcc-s1 lib32stdc++6
+
+sudo mkdir -p /opt/steamcmd /opt/l4d2/{installation,overlays,instances,runtime}
+sudo chown -R "$USER:$USER" /opt/steamcmd /opt/l4d2
+sudo loginctl enable-linger "$USER"
+```
+
+SteamCMD should be installed so the runtime user can update it. If installing from Valve's tarball, avoid symlinking `steamcmd.sh` directly because it derives its install root from `$0`. Use a wrapper instead:
+
+```bash
+curl -fsSL https://steamcdn-a.akamaihd.net/client/installer/steamcmd_linux.tar.gz -o /tmp/steamcmd_linux.tar.gz
+tar -xzf /tmp/steamcmd_linux.tar.gz -C /opt/steamcmd
+sudo tee /usr/local/bin/steamcmd >/dev/null <<'EOF'
+#!/bin/sh
+exec /opt/steamcmd/steamcmd.sh "$@"
+EOF
+sudo chmod 755 /usr/local/bin/steamcmd
+chmod 755 /opt/steamcmd/steamcmd.sh /opt/steamcmd/linux32/steamcmd
+steamcmd +quit
+```
+
+`uv` is optional deployment tooling. Debian 13 did not provide an `uv` package during the smoke test, so install it explicitly if you want to use it for faster virtualenv/dependency setup. `l4d2ctl` does not require `uv` at runtime.
+
 ## Web App Read APIs
 
 These read APIs are provided for web app integration: