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/.

modding/mdlp-image/stage-hacked.sh

@@ -0,0 +1,20 @@
+#!/usr/bin/env bash
+# Stages a scratch, patched copy of src/hacked/ at the given destination -
+# shared by process-mods.sh (mdlp) and ../gen_animgallery.sh (animgallery),
+# both of which build against hacked's source directly (via their own
+# go.mod's `replace ../hacked`) and so both need the same patches applied
+# that hacked-build-image/build-hacked.sh already applies for hacked's own
+# binary. src/hacked/ is never built or patched in place.
+#
+# Usage: stage-hacked.sh <modding-root> <dest-dir-for-hacked>
+set -euo pipefail
+
+MODDING_ROOT="$1"
+DEST="$2"
+
+rm -rf "$DEST"
+mkdir -p "$(dirname "$DEST")"
+cp -a "$MODDING_ROOT/src/hacked" "$DEST"
+for p in "$MODDING_ROOT"/src/hacked-patches/*.patch; do
+    patch -p1 -d "$DEST" < "$p"
+done