Textures: nine-slice scale per texture (#96); Repeat is the default fill

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Development.md

@@ -12,7 +12,7 @@ npm run check        # typecheck + lint + format:check + tests, the gate CI runs
                      #   code is lost (tail's is reported), so a red run reads as green
 npm run build        # writes dist/ and regenerates index.html
 ./build.sh           # the same under `set -o pipefail`, for callers that pipe the output
-node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 958 editor checks, screenshots in /tmp/gm-e2e-*.png
+node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 960 editor checks, screenshots in /tmp/gm-e2e-*.png
 GM_STEPS=20 node tools/e2e.mjs                                # or 1-12, or 6,20 — a subset while working on one step
 ./e2e.sh start [GM_STEPS]                                     # the same run detached, see below
 ./e2e.sh status                                               # live progress, or the last run's result
@@ -232,4 +232,4 @@ widths to find click points. The **render device is not in this repo** — `/dev
 and a 1 GB `/dev/shm` come from `container.options` in the runner's own `config.yaml`, which
 applies to every job on that runner. Without them chromium falls back to llvmpipe, which gets
 WebGL 1.0 only. Measured in the job container: `ANGLE (Intel, Vulkan 1.3.230, HD Graphics 630)`,
-WebGL 2.0, and the same 958 checks as here.
+WebGL 2.0, and the same 960 checks as here.

Textures.md

@@ -86,8 +86,9 @@ whole inside of a frame does not swallow the lines it holds. **All four at zero
 not sliced at all** — it is drawn over the whole box, exactly as every sprite was before #41, and a
 frame nobody sliced carries no border keys in the saved document.
 
-**Fill** says how the parts *between* the corners fill their space: `Stretch` pulls them, `Repeat`
+**Fill** says how the parts *between* the corners fill their space: `Repeat` tiles them, `Stretch`
-tiles them. One setting for the edges and the middle together, because a brick pattern that tiles
+pulls them. `Repeat` is the default, so only a `Stretch` is written into the saved document.
+One setting for the edges and the middle together, because a brick pattern that tiles
 along the top and stretches in the centre reads as a mistake. A tiled part is built as one quad per
 tile, each with the frame's own UVs and the last one cut off where the space ends — sampling past
 1 with a repeating address mode would pull in the *neighbouring cell of the sheet*, which is the
@@ -100,6 +101,15 @@ corners meet at a seam. And a frame can never be given borders that leave *no* m
 squeezed to leave at least one pixel, because a middle of no pixels has nothing to draw the space
 between the corners with, and that would show as a hole rather than as an error.
 
+**Nine-slice scale** is a property of **the texture**, not of a frame: it says how many sprite units
+one border pixel is drawn with, for every sliced frame the texture has. The default 1 draws a 16 px
+border 16 units wide; at 2 the same border is 32 units, at 0.5 it is 8. The pixels a corner shows
+stay the same — only its size on screen changes — so a pixel-art button can have chunky corners
+without redrawing the sheet at twice the size. With `Repeat` the tiles scale along, because a border
+drawn twice as big beside a pattern at its old size reads as two different pictures. The rule for a
+sprite narrower than its borders applies to the scaled size: two 32-unit borders on a 20-unit box
+become two of 10.
+
 **Fitting the sprite to the picture.** The *Width* and *Height* rows of a sprite carry a ↔ / ↕
 button offering two things: **Fit to 64 x 48** sets that one side from the other and the picture's
 proportions — the frame's rectangle when it wears a frame, the whole image otherwise — and **Set