pack-dist: ship steam_appid.txt so a fresh dist is test-ready

The packer wipes and rebuilds dist\, which silently discarded the
hand-made steam_appid.txt every repack - and without it SteamAPI_Init
fails and the game quietly falls back to TCP, which is the confusing
symptom testers hit. The dist now ships it (480, Spacewar) alongside
the RP412STEAM=1 environ.ini, so copying the folder to a machine with
Steam running is the whole setup. Test doc updated to match.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/STEAM-3-MACHINE-TEST.md

@@ -9,13 +9,13 @@ the first live test.
 1. Steam client installed, logged in (different account per machine),
    and RUNNING.
 2. Copy the `dist\` folder onto the machine.
-3. In the dist folder, create `steam_appid.txt` containing exactly:
-   `480`
-   (Spacewar, Valve's public dev AppID — replaced by the real AppID
-   once the store page exists.)
-4. Add to `environ.ini`:
-   `RP412STEAM=1`
-5. Start with `start-windowed.bat` as usual.
+3. Start with `start-windowed.bat` as usual.
+
+The packed dist already contains `steam_appid.txt` (480 — Spacewar,
+Valve's public dev AppID, replaced by the real one once the store page
+exists) and ships `RP412STEAM=1` in `environ.ini`. If a machine has an
+older dist, check both before blaming the network: a missing appid
+file makes SteamAPI_Init fail and the game quietly falls back to TCP.
 
 Sanity check per machine: `rpl4.log` should show
 `SteamNetTransport: up as 169.254.x.y (fake ports N console, M game)`.

pack-dist.ps1

@@ -47,6 +47,13 @@ if (Test-Path $pdb) { Copy-Item $pdb $dist }
 # only activates with RP412STEAM=1; plain desktop runs never touch it.
 Copy-Item (Join-Path $root 'extern\steamworks_sdk_164\sdk\redistributable_bin\steam_api.dll') $dist
 
+# steam_appid.txt: until RP412 has its own AppID, Steam testing runs
+# under Spacewar (480). Without this file SteamAPI_Init fails and the
+# game falls back to plain TCP - which is exactly the confusing symptom
+# testers hit when the file goes missing. Delete it once we ship under
+# our own AppID (the Steam client provides it then).
+Set-Content -Path "$dist\steam_appid.txt" -Encoding ascii -Value '480'
+
 # --- game data -------------------------------------------------------------
 foreach ($dir in 'AUDIO', 'GAUGE', 'VIDEO') {
     Write-Host "  copying $dir..."