Bake stub atlas-sets for demo_tilemap and walls_tilemap

Replaces the legacy color-only *.tilemap.json fixtures with baked
2-tile atlas-sets in the new atlases/<id>/{diffuse,height,atlas-json,
lock-json} layout. Stub PNGs are 32x32 solid colors matching the
previous color values; height channel is synthesized L8=0 by the baker.
A scripts/bake.sh re-bake helper drives the atlas-baker tool against
assets/_sources/.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/bake.sh

@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+# Re-bake the stub atlases used by maps-test fixtures.
+set -euo pipefail
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+LIB_DIR="$(cd "$SCRIPT_DIR/.." && pwd)"
+BAKER=~/Projects/Sporel/sporel-tool-atlas-baker/bin/atlas-baker.js
+
+node "$BAKER" \
+    --in  "$LIB_DIR/assets/_sources/demo_tilemap" \
+    --out "$LIB_DIR/assets/atlases/demo_tilemap" \
+    --atlas-id demo_tilemap \
+    --tile-size 32
+
+node "$BAKER" \
+    --in  "$LIB_DIR/assets/_sources/walls_tilemap" \
+    --out "$LIB_DIR/assets/atlases/walls_tilemap" \
+    --atlas-id walls_tilemap \
+    --tile-size 32 \
+    --blocks-sight-pattern '^wall_'