Add engine-debug-tools and fix critter animation rendering bugs

- engine-debug-tools/: a debug dump hotkey + dump2png to inspect
  decoded critter frames independent of the renderer (see root
  README.md's "Debugging" section).
- engine-patches/04,05: fix critter sprite scale and an OpenGL
  texture cache collision, found using that tooling.
- mdlp: fix encodeFrames RLE-diffing upscaled frames against a
  previous-frame reference the real RSD8 decoder never implements
  (verified against Night Dive's original Mac source down to the
  68K assembly) - was rendering as scattered static in-game.
- Upscale pipeline: better color quantization (dithering, pure-ramp
  exclusion) and per-resource model selection.

modding/mdlp-image/Dockerfile

@@ -21,23 +21,43 @@ RUN apt-get update \
     && DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
         ca-certificates curl gosu patch python3 python3-pip \
     && rm -rf /var/lib/apt/lists/*
-RUN pip3 install --no-cache-dir onnxruntime pillow numpy
+RUN pip3 install --no-cache-dir onnxruntime pillow numpy scipy
 
-# The AI upscaler model for mdlp's upscale mod type (see ../README.md
-# section 4): RealESRGAN_x4plus_anime_6B, a 6-block RRDBNet ONNX-exported
-# from xinntao/Real-ESRGAN's anime-illustration checkpoint (BSD-3-Clause).
-# It's trained on flat-color line art, so it keeps hard sprite edges rather
-# than smoothing them the way a photo- or video-denoising model would. A
-# mod's own "model" field (upscaleDef in ../src/mdlp/upscale.go) is not
-# consulted; every resource goes through this one model. upscale_batch.py
-# resizes its fixed native 4x output down to whatever scale a mod actually
-# asks for.
-ARG UPSCALE_MODEL_URL=https://huggingface.co/skillsafe-ai/realesrgan-x4plus-anime-6b/resolve/main/model.onnx
-RUN mkdir -p /opt/upscale-model \
-    && curl -sSL -o /opt/upscale-model/model.onnx "$UPSCALE_MODEL_URL"
+# The AI upscaler models for mdlp's upscale mod type (see ../README.md
+# section 4), selected per mod by a bitmaps/*.json file's own "model" field
+# (upscaleDef in ../src/mdlp/upscale.go) - upscale_batch.py resolves a name
+# straight to $UPSCALE_MODELS_DIR/<name>.onnx, and picks tiled or single-pass
+# inference based on each model's own declared ONNX input shape (fixed vs
+# dynamic height/width), so adding another model here needs no code change.
+# Each model's RGB output is re-quantized to SS1's fixed 256-color palette
+# with Floyd-Steinberg dithering (imageToBitmap in ../src/mdlp/upscale.go),
+# not a plain nearest-color map - without it, any model's smooth gradients
+# can land on a visibly wrong, much darker palette entry in a solid patch
+# rather than unnoticeable dithering noise.
+# All three are BSD-3-Clause ONNX exports of xinntao/Real-ESRGAN checkpoints:
+# - realesrgan-x4plus-anime: a 6-block RRDBNet trained on flat-color
+#   illustration, keeping hard sprite edges rather than smoothing them the
+#   way a photo- or video-denoising model would. Dynamic input shape. Default
+#   choice - sharpest result on most sprites.
+# - realesrgan-x4plus: the general-purpose photo checkpoint (16.7M-param
+#   RRDBNet). Fixed 128x128 input, needs tiling for anything larger, and
+#   much slower than the other two. Worth trying when the anime model
+#   mis-renders a specific sprite, but can itself mis-render different ones.
+# - realesr-general-x4v3: a compact 1.2M-param SRVGGNet, dynamic input
+#   shape, ~100x faster than realesrgan-x4plus. Noticeably softer than
+#   either RRDBNet model, but the most robust against both models' failure
+#   modes - the model to reach for on a specific sprite neither RRDBNet
+#   model renders cleanly, not as a blanket replacement.
+ARG UPSCALE_MODEL_ANIME_URL=https://huggingface.co/skillsafe-ai/realesrgan-x4plus-anime-6b/resolve/main/model.onnx
+ARG UPSCALE_MODEL_GENERAL_URL=https://huggingface.co/qualcomm/Real-ESRGAN-x4plus/resolve/6f5749d5adff39de0127e42bc45c8ec8e6f76a66/Real-ESRGAN-x4plus.onnx
+ARG UPSCALE_MODEL_COMPACT_URL=https://huggingface.co/jonathanst29/tinier-upscale-models/resolve/main/realesr-general-x4v3.onnx
+RUN mkdir -p /opt/upscale-models \
+    && curl -sSL -o /opt/upscale-models/realesrgan-x4plus-anime.onnx "$UPSCALE_MODEL_ANIME_URL" \
+    && curl -sSL -o /opt/upscale-models/realesrgan-x4plus.onnx "$UPSCALE_MODEL_GENERAL_URL" \
+    && curl -sSL -o /opt/upscale-models/realesr-general-x4v3.onnx "$UPSCALE_MODEL_COMPACT_URL"
 COPY mdlp-image/upscale_batch.py /usr/local/bin/upscale_batch.py
 RUN chmod 755 /usr/local/bin/upscale_batch.py
-ENV UPSCALE_MODEL_PATH=/opt/upscale-model/model.onnx
+ENV UPSCALE_MODELS_DIR=/opt/upscale-models
 
 RUN curl -sSLO "https://go.dev/dl/go${GO_VERSION}.linux-amd64.tar.gz" \
     && tar -C /usr/local -xzf "go${GO_VERSION}.linux-amd64.tar.gz" \