Claimed floors back to fitted flagstones in the masters' colours with damage; high-res floor decals

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

art/generated/textures/decals/PROMPTS.md

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+# Floor decals (high-res)
+
+Wear and tear the game lays over some plain floor tiles (`D_RUBBLE`, `D_CRACK`; `HD_DECALS` in
+`src/gfx.rs`). Generated on 2026-11-07 with `fal-ai/nano-banana-pro` (`aspect_ratio: 1:1`,
+`resolution: 1K`), the decal's sentence followed by the shared text:
+
+```text
+Seen straight from above (orthographic top-down, no perspective), flat even lighting, no cast shadows. Hand-painted pixel art style for a dark-fantasy dungeon game, crisp edges, readable at small size. The background is a completely flat pure magenta (#FF00FF) everywhere else - no floor, no stones, no texture behind it, no frame, no text.
+```
+
+## rubble
+
+```text
+An overlay decal for a dungeon floor: a loose scatter of small grey-brown pebbles, stone fragments and a little grit and dust, spread unevenly over the whole square image, denser in one corner, about thirty small pieces of different sizes, each with a dark outline.
+```
+
+## crack
+
+```text
+An overlay decal for a dungeon floor: one long dark jagged crack running diagonally across the whole square image from corner to corner, with a few short branching side cracks and two or three tiny chipped splinters beside it; only the dark crack lines themselves.
+```
+
+## Processing
+
+`tools/make_decal.py` keys out the magenta and scales down with hard alpha; the crack is made
+neutral grey (its lines took on the magenta):
+
+```bash
+python3 tools/make_decal.py rubble-raw.png rubble --size 128   # and --size 64
+python3 tools/make_decal.py crack-raw.png crack --size 128 --gray
+```
+
+Copied to `assets/textures/decal-<name>.png` (64) and `decal-<name>-128.png`.