Pivot to the original Raptor source as the engine base

Vendor skynettx/raptor (nukeykt's reconstructed source, GPLv2+): original
game logic, GLB loading, OPL2 music emulation. Runs with user-supplied
registered v1.2 GLB data (5 original mission packs) at 1920x1080 with
working AdLib soundtrack and SFX.

- play.sh: asset linking, first-run build, launch
- SDL2 2.28.2 built user-local (~/.local); CMake 3.30.3 binary
- prototype engine preserved on branch prototype-engine
- PLAN.md rewritten for the source-base architecture

.gitignore

@@ -1,8 +1,16 @@
-build/
-build-*/
-.cache/
-compile_commands.json
+# Original game data (copyrighted — user-supplied, never committed)
+original/
+game/
+*.GLB
+*.zip
+
+# Vendored raptor build output
+raptor/build/
+
+# Legacy prototype build output
+bin/
 *.o
-*.obj
-*.exe
-.DS_Store
+*.d
+
+# Local toolchain
+.local/