Add AI-upscaled critter/object art as a new MDL mod type

Extends the MDL system beyond text overrides: bitmaps/<label>.json
names a model/scale and resource list, and mdlp extracts each
resource's frames, runs them through an ONNX Runtime CPU upscaler
baked into mdlp-image, re-quantizes to the game's palette, and writes
the result back - same build-time pipeline as message overrides.

Adds animgallery, a standalone viewer that renders any data
directory's animated resources as an HTML/GIF gallery with a
checkbox-driven MDL JSON generator, and -only-changed to preview just
what a mod overrides against the base game.

Fixes a real data race in hacked's ChangeNotifier.hashAll() (found via
go build -race causing non-deterministic resource counts), via a new
numbered patch rather than editing the vendored source.

Adds engine-patches/, a general-fixes counterpart to
android/engine-patches/ now staged and applied to the desktop builds
too (previously unpatched): a real GL_UNPACK_ROW_LENGTH stride bug, an
audio error-logging improvement, and two critter-rendering fixes -
mod art was losing to the base game's own re-opened resource files,
and critters rendered at the wrong size/position once that was fixed,
both from how the billboard renderer derives on-screen size and
placement straight from bitmap pixel dimensions.

Updates README.md and AGENTS.md to document engine-patches/.

engine-patches/02-desktop-audio-error-detail.patch

@@ -0,0 +1,11 @@
+--- a/src/MacSrc/SDLSound.c	2026-07-19 13:11:51.000000000 +0200
++++ b/src/MacSrc/SDLSound.c	2026-09-27 11:42:37.969726579 +0200
+@@ -42,7 +42,7 @@
+     }
+ 
+     if (Mix_OpenAudio(48000, AUDIO_S16SYS, 2, 2048) < 0) {
+-        ERROR("%s: Couldn't open audio device", __FUNCTION__);
++        ERROR("%s: Couldn't open audio device: %s", __FUNCTION__, SDL_GetError());
+     }
+ 
+     Mix_AllocateChannels(SND_MAX_SAMPLES);