Shader preview sliders show again after a property edit (#168)

Changing min, max or default of a property rebuilt the slider rows of
the shader preview, but the rows stayed at 0 x 0. Only the labels were
visible until the preview was resized by hand.

- Widget.resize only lays out when the size changes. The row count
  stays the same on such an edit, so the row stack kept its height and
  never laid out its new children.
- syncParams now relayouts the row stack itself after the column
  around it.

Tests:
- e2e 28n: a new max rebuilds the slider at 136 x 24, a click at its
  right end sets level 2. Red against the old build (w 0, h 0).
- GM_STEPS=28,31 fresh: ALL PASSED, 106 checks (was 105)
- ./check.sh: 132 files, 1952 tests, exit 0
- Check sum 1210 in README, computed (1209 + 1), CI measures it.

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

README.md

@@ -43,7 +43,7 @@ npm run build        # writes dist/ and regenerates index.html; the folder is wh
 The browser suite and the three example games need a machine with a GPU:
 
 ```bash
-node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1209 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1210 checks (GM_STEPS=20 for one step)
                                                    #   every tool signs in first – see the Accounts page
 node tools/seed-jumprun.mjs && node tools/smoke-jumprun.mjs
 node tools/seed-tetris.mjs && node tools/smoke-tetris.mjs