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

play.sh

@@ -0,0 +1,27 @@
+#!/bin/sh
+# Launch Raptor: Call of the Shadows (original engine + original data).
+# Requires: SDL2 (user-local build OK at ~/.local), GLB files in original/.
+set -e
+cd "$(dirname "$0")"
+
+if [ ! -d original ]; then
+    echo "Extract the original game archive first:" >&2
+    echo "  unzip 'Raptor Call of the Shadows 2002 Registered.zip' -d original" >&2
+    exit 1
+fi
+
+mkdir -p game
+for i in 0 1 2 3 4; do
+    ln -sf "$PWD/original/Raptor Call of the Shadows/FILE000$i.GLB" "game/FILE000$i.GLB" 2>/dev/null
+done
+
+if [ ! -x game/raptor ]; then
+    echo "Building raptor (first run)..." >&2
+    cmake -E make_directory raptor/build
+    ( cd raptor/build && cmake .. -DCMAKE_BUILD_TYPE=Release ${CMAKE_PREFIX_PATH:+-DCMAKE_PREFIX_PATH=$CMAKE_PREFIX_PATH} && make -j"$(nproc)" )
+    cp raptor/build/bin/raptor game/
+fi
+
+export LD_LIBRARY_PATH="${LD_LIBRARY_PATH:+$LD_LIBRARY_PATH:}$HOME/.local/lib"
+cd game
+exec ./raptor