Keep edited list values when an item is deleted (#235)

The list tab read the list once when it built its rows. Editing a
value does not rebuild the rows, so the delete button later filtered
that old copy and wrote it back. Every edit since the last rebuild was
lost. Adding an item only triggered it, because adding rebuilds the
rows and that state became the old copy.

- ListAccess.value in src/editor/list/slot.ts is now read on every
  access instead of once when the access is built.
- The kind switch of a row and the struct field buttons (field type,
  rename, remove field, + Field) read the item when clicked, not
  when the row was built. They had the same stale copy.
- New test hook __gm.list.removeAt(i) clicks the delete button of one
  row, because the tab shows many buttons with the same label.

Not covered by a test of their own: the struct field buttons and the
kind switch after an edit. The cause is the same, the fix is in code.

Tests:
- ./check.sh: 2435 tests, CHECK EXIT 0.
- Unit test in tests/editor/list-slot.test.ts keeps one access, edits
  a value, adds and edits an item, deletes another. All edits stay.
  It fails without the fix.
- e2e 21b-ii with real clicks: set row 2 to 66, delete row 0, expect
  ["11","66"], two undos take both back. e2e 21: 52 checks passed.
- Counter probe: with the old snapshot the check fails with
  ["11","44"], the reported bug.
- Check total in README is now 1400 (+2), counted, not measured.

Co-Authored-By: Claude Opus 5.5 <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, 1398 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1400 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