Describe the two drawing layers of the texture tab (#220)

Architecture explains the sheet and overlay layers, the pan without rebuild and the measured times. Development has the new check total 1336.

Architecture.md

@@ -287,6 +287,18 @@ last pointer move a few milliseconds later to update hover, with the same native
 each one ran a full preview. Inspector scrubbing and the gradient bar's colour stops still preview
 on every move.
 
+The Texture tab draws its frames in two layers (#220). `sheet` holds every frame but the selected
+one with its outlines, `overlay` the selected frame, its handles and a frame being spanned. Both
+are drawn without the pan, in a container that sits at the pan. A pan only moves that container,
+and only the names (which stay on frames in view) are placed anew. `sheet` is rebuilt only when
+the zoom, the selection or one of its frames changes. A frame's outline list stands for its
+corners, because the model replaces the list instead of editing it. Whether an outline is a
+polygon is remembered per list too; it used to be worked out on every redraw by cutting the
+outline into convex pieces. With 1728 frames of one outline each (768 x 576 px, 16 px grid), per
+pointer move on the main thread: pan 77 → 5 ms, dragging a frame 52 → 10 ms, a wheel step
+52 → 31 ms, selecting another frame 68 → 41 ms. The last two rebuild `sheet` on purpose, once
+per step or click. `__gm.texture.drawInfo()` counts the rebuilds for the checks.
+
 The toolbar's right edge shows the frame rate in edit and play mode alike; `__gm.fps()` exposes
 the same number to tests. It counts **drawn** frames over the time the renderer was **awake**:
 idle seconds are not slow seconds, so the readout answers "how smooth is it while it moves" and

Development.md

@@ -14,7 +14,7 @@ npm run build        # writes dist/ and regenerates index.html
 ./build.sh           # the same under `set -o pipefail`, for callers that pipe the output
 php tools/make-user.php <name> <password> [admin|user]        # an account; every tool signs in (see Accounts)
 php tests/php/accounts.test.php                               # the API's account rules, no server needed (CI's gate job)
-node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1333 editor checks, screenshots in /tmp/gm-e2e-*.png
+node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1336 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