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/.

build-image/build-engine.sh

@@ -8,16 +8,41 @@
 # container - see the Makefile's `engine` target, which picks whichever
 # of these applies).
 #
+# Builds from a scratch copy of engine/, patched with engine-patches/
+# (see that directory and README.md section 4.4) - same reasoning as
+# build-engine-win.sh's own scratch copy: engine/ itself is never modified in
+# place (README.md section 1).
+#
 # Output lands in engine/.build-output/: the systemshock binary, the
 # shared libraries it needs at runtime (lib/), and a default MIDI
 # soundfont - everything the root Makefile needs to assemble dist/.
 set -euo pipefail
 
 REPO_ROOT="$(pwd)"
-ENGINE_DIR="$REPO_ROOT/engine"
-OUT_DIR="$ENGINE_DIR/.build-output"
+SCRATCH_ENGINE="$REPO_ROOT/build/linux-engine"
+OUT_DIR="$REPO_ROOT/engine/.build-output"
 
-cd "$ENGINE_DIR"
+echo "== Preparing a scratch copy of engine/, patched with engine-patches/ =="
+rm -rf "$SCRATCH_ENGINE"
+mkdir -p "$(dirname "$SCRATCH_ENGINE")"
+cp -a "$REPO_ROOT/engine" "$SCRATCH_ENGINE"
+
+# A checkout built in place by an older version of this script (before it
+# staged a scratch copy) can still have build_ext/CMakeCache.txt etc.
+# sitting in the real engine/ - copied along above, they'd make CMake
+# refuse to configure this copy ("directory is different than the
+# directory where CMakeCache.txt was created"). Same cleanup
+# build-engine-win.sh's own scratch copy does.
+rm -rf "$SCRATCH_ENGINE"/build_ext "$SCRATCH_ENGINE"/CMakeCache.txt \
+    "$SCRATCH_ENGINE"/CMakeFiles "$SCRATCH_ENGINE"/cmake_install.cmake \
+    "$SCRATCH_ENGINE"/Makefile "$SCRATCH_ENGINE"/systemshock \
+    "$SCRATCH_ENGINE"/src/Libraries/CMakeFiles
+
+for p in "$REPO_ROOT"/engine-patches/*.patch; do
+    patch -p1 -d "$SCRATCH_ENGINE" < "$p"
+done
+
+cd "$SCRATCH_ENGINE"
 
 echo "== Wiring up prebuilt SDL2/SDL2_mixer/fluidsynth-lite from the image =="
 rm -rf build_ext