feat: collect id tech patches and build demos

emulators/dhewm3.yml

@@ -15,8 +15,8 @@ notes: |
   Doom 3 engine (idTech 4). Carries the original Doom 3 1.3.1 game
   code plus bug fixes, SDL abstraction and 64-bit cleanup.
 
-  The engine refuses to run without a `base/` directory containing
-  the retail pak files. Resurrection of Evil goes in `d3xp/`.
+  Game data is read from the pak files of `base/`, or from `demo/` when
+  base/ yields no default.cfg. Resurrection of Evil goes in `d3xp/`.
   ref: neo/framework/FileSystem.cpp:88-112 ("either base or demo"),
        neo/framework/FileSystem.cpp:2089 AddGameDirectory,
        neo/framework/FileSystem.cpp:2235 SetupGameDirectories(BASE_GAMEDIR)
@@ -26,6 +26,12 @@ notes: |
   order so a later pak overrides earlier ones.
   ref: neo/framework/FileSystem.cpp:2125-2143 (ListOSFiles ".pk4").
 
+  When default.cfg cannot be read and fs_game is unset or "base", the
+  engine sets fs_game and fs_game_base to "demo" and restarts the file
+  system on the demo data. With any other fs_game it stops with
+  "Couldn't load default.cfg".
+  ref: neo/framework/FileSystem.cpp:2703-2716
+
 files:
   # ================================================================
   # base/ - Doom 3 retail paks
@@ -47,6 +53,7 @@ files:
     path: "base/pak001.pk4"
     required: true
     source_ref: "neo/framework/FileSystem.cpp:2125"
+    unsourceable: "retail data the collection does not hold; a same-named file of another game must not stand in"
 
   - name: "pak002.pk4"
     category: game_data
@@ -120,3 +127,23 @@ files:
     required: false
     source_ref: "neo/framework/FileSystem.cpp:3744-3754 (explicit d3xp/pak000.pk4 probe)"
     note: "Resurrection of Evil main pak."
+
+  - name: "pak001.pk4"
+    category: game_data
+    system: doom3
+    path: "d3xp/pak001.pk4"
+    required: false
+    source_ref: "neo/framework/FileSystem.cpp:2125"
+    note: "Resurrection of Evil patch pak, shipped by the 1.3.1 patch."
+
+  # ================================================================
+  # demo/ - Doom 3 demo, read when base/ holds no default.cfg
+  # ================================================================
+
+  - name: "demo00.pk4"
+    category: game_data
+    system: doom3
+    path: "demo/demo00.pk4"
+    required: false
+    source_ref: "neo/framework/FileSystem.cpp:2703-2709 (demo/ fallback), 2125"
+    note: "Doom 3 demo data. Every .pk4 of demo/ is loaded once the fallback sets fs_game to demo."