Add complete game working copy under assets/ (runtime data + binaries)

Imports the full shipped working copy of Red Planet v4.10 so the repo is
self-contained: buildable (source) and runnable (data). ~110 MB, committed
directly to git per project decision.

Contents (assets/RP411/):
- AUDIO/  (~97 MB) 223 .wav + audio config/res
- VIDEO/  (~10 MB) .X meshes, .SKL/.det/.sph geometry, .met materials,
          .pcc/.png/.pcx textures, .vsh/.psh shaders, material tables
- GAUGE/  (~0.8 MB) HUD .GIM images, .PCC/.PCX bitmaps, gauge config
- SPOOLS/ runtime replay output dir (kept via .gitkeep)
- Root config/launch: environ.ini, RPDPL.INI, JOYSTICK.INI, TEST.EGG,
  RPL4.RES, *.bat launchers
- Runtime binaries: rpl4opt.exe (shipped reference build), libsndfile-1.dll,
  oalinst.exe (OpenAL redist installer), sleep.exe

Housekeeping:
- .gitignore: re-include assets/**/*.exe and *.dll (global rules skip them);
  keep Thumbs.db and runtime *.spl out.
- .gitattributes: mark asset media (.wav/.png/.pcc/.gim/.x/.skl/... ) binary.
- docs/ASSETS.md: runtime layout, config chain (environ.ini -> RPDPL.INI ->
  video/audio/gauge paths), launch args, and per-directory asset inventory.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -52,3 +52,11 @@ rpl4.log
 Thumbs.db
 desktop.ini
 .DS_Store
+
+# Bundled game working copy: committed wholesale as runtime data (see
+# docs/ASSETS.md). Re-include the binaries the global rules above would skip.
+!assets/**/*.exe
+!assets/**/*.dll
+# ...but never the spool runtime output or Windows thumbnail caches.
+assets/**/*.spl
+assets/**/last.spl