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/.

AGENTS.md

@@ -28,7 +28,8 @@ These come from the maintainer. Follow them without being asked again.
 1. **Never commit.** Do not run `git commit` (or amend, rebase, push), even if
    asked to commit. Draft the commit message and show it; the maintainer runs
    the commit. Mention any new untracked file that has to be `git add`ed
-   (typically a new patch under `android/engine-patches/`).
+   (typically a new patch under `android/engine-patches/` or
+   `engine-patches/`).
 2. **Do not build.** Do not run `make apk`, `./gradlew`, or `make android-studio`
    to check that a change compiles. The maintainer deploys from Android Studio.
    `make apk` and `make android-studio` overwrite `android/engine.properties`
@@ -38,7 +39,9 @@ These come from the maintainer. Follow them without being asked again.
    tested.
 3. **`engine/` and `android/gl4es-src/` are never edited.** They are vendored
    snapshots. Every change to them is a numbered patch under
-   `android/engine-patches/` or `android/gl4es-patches/`, applied to a scratch
+   `android/engine-patches/` or `android/gl4es-patches/` (Quest-specific
+   fixes) or `engine-patches/` (general fixes, applied to the desktop
+   Linux/Windows builds too - README section 4.4), applied to a scratch
    copy at build time. (`engine/` is GPLv3, see README section 6.)
 4. **Patch numbers stay contiguous** (`01`, `02`, ... with no gap). When you
    insert or delete a patch, renumber the rest and update every reference to
@@ -68,6 +71,7 @@ These come from the maintainer. Follow them without being asked again.
 |---|---|
 | `engine/` | Vendored Shockolate source. Read-only for us. |
 | `android/engine-patches/` | Numbered `patch -p1` files applied to a scratch copy of `engine/`. |
+| `engine-patches/` | Same, but general fixes applied to the desktop Linux/Windows builds too (README section 4.4), not Quest-specific ones. |
 | `android/gl4es-src/`, `android/gl4es-patches/` | Vendored GL4ES (desktop GL to GLES) and its patches. |
 | `android/app/src/main/cpp/` | First-party native code (section 4). |
 | `android/app/src/main/java/de/ladkau/questshock/` | First-party Java (section 4). `org/libsdl/app/` is vendored SDL2 glue (zlib). |
@@ -185,6 +189,14 @@ running" and gates the cheats and the menu pause.
 
 ## 6. Engine patches
 
+This section is written for `android/engine-patches/`; `engine-patches/`
+(general fixes, also applied to the desktop Linux/Windows builds - README
+section 4.4) follows the same process below except: no `#ifdef __ANDROID__`
+guard (these aren't Android-specific), save as
+`engine-patches/NN-desktop-<topic>.patch`, and there is no Gradle
+`stageEngine` to worry about going stale - `build-image/build-engine.sh`/
+`build-engine-win.sh` re-stage and re-patch on every build.
+
 Rules:
 
 - Guard every engine change with `#ifdef __ANDROID__` so the desktop build is