Fix animgallery rendering bugs and mitigate AI-upscale frame flicker

- writeGIF now pads every frame to one shared canvas instead of
  writing each at its own (possibly storage-cropped) size - GIF
  disposal rules were showing a previous frame's stale pixels through
  the uncovered border of a smaller frame, bleeding content between
  frames in the gallery.
- decodeFrames/decodeViewFrames/decodeAnimationFrames now always hand
  bitmap.DecodeReferenced a zero buffer instead of the previous
  frame/block's real pixels. The game's actual RSD8 decoder treats a
  "skip" run as literal zero, with no mechanism to carry a reference
  frame's pixels forward (confirmed against Night Dive's published
  source), and hacked's own multi-block Bitmap preview decodes the
  same way. The previous behavior ghosted a previous frame's stale
  content into the current frame's skip-run regions, in both the
  gallery and the AI upscaler's source input.
- index.html now shows a labeled per-frame filmstrip next to each
  animated GIF, and a base-vs-this-build side-by-side comparison for
  every resource a mod overrides.
- Gallery GIFs play back at a third of real speed by default, for
  easier frame-by-frame review.
- modding/mods/*/bitmaps/*.json gains considerPreviousFrames: a
  per-resource list that runs a temporal-consistency pass (median
  across the animation's own frames) on independently-upscaled frames
  to reduce per-frame AI flicker. Applied to 0x7B1 in upscale_medbot.
- Drop the rebecca-test example mod.

modding/Makefile

@@ -18,7 +18,9 @@
 #   animgallery_mods - same, but with every built mod under dist/mods/
 #                       layered on top (later wins), in build/gallery-mods/ -
 #                       shows what a mod actually changes without launching
-#                       the game. Run `make mods` first.
+#                       the game. Every card for a resource a mod overrides
+#                       shows the original alongside it for comparison. Run
+#                       `make mods` first.
 #   animgallery_dist - every built mod under dist/mods/ layered on top of
 #                       the base game, like animgallery_mods, but restricted
 #                       to only the resources a mod actually overrides, in
@@ -29,6 +31,7 @@
 #                       Run `make mods` first.
 #                       (animgallery_base/_mods/_dist all write into this
 #                       subproject's own build/, not the repo root's.)
+#
 #   clean            - removes dist/.
 .PHONY: all build-image hacked mdlp-image mods animgallery_base animgallery_mods animgallery_dist clean