Add row groups to item scanning

[enaboapps]

Closes #1019.

Summary

• New Item scanning choice: Rows, groups, then items sits between Groups, then items and One item at a time. It can be set as the shared default or for one area (menus, keyboard, mouse panel).
• Splitting, as Android's Group Scan (RowGroupingPolicy) does: a row of 7 or more items becomes round(√n) groups, between 2 and 4, with at least 2 items each, sized evenly with larger groups first. For example, 10 keys split 4, 3, 3. Shorter rows scan as before. Android's gap-aware cut is left out because panel keys have no gaps.
• Scanning order: row, then group, then item. Groups are identified scan_tree groups, so their highlight survives the keyboard replacing its rows.
• Panels:
  • The keyboard and mouse panel highlight the whole group with the row style and show "Select a group".
  • After a group's last item, Back to groups highlights the group and returns to it. After a row's last group, Back to rows works as before.
• Menus: they have at most three columns, so they never split.

Settings compatibility

Local settings file. The choice is stored as pattern: "grouped" plus a new rowGroups boolean, not a new pattern value. Versions rc.17 to rc.23 only drop unknown values of panelSize, nextScan and startFrom. An unknown pattern would make them reset every scan preference to defaults.

• Older versions ignore rowGroups and read the pattern as Groups, then items.
• Existing settings are unchanged: rowGroups defaults to off. An area with its own pattern saved without rowGroups has no row groups.
• Choosing an area's default clears both values. Point scanning ignores row groups.
• Unknown pattern or rowGroups values are now dropped on their own when settings are read; the other scan settings are kept.

Settings sync. Synced documents must round-trip exactly, so portable_settings::SCHEMA_VERSION goes from 2 to 3.

• upgrade_v2 reads version 2 documents with rowGroups: false and every area following the shared choice. Version 1 documents upgrade through both steps.
• Builds on version 2 report a version 3 document as Update required rather than invalid. They never overwrite it.
• As with the version 1 to 2 bump, a merge base saved by an older build doesn't cause an upload by itself.

Validation

• npm run prediction-model, npm run lint, npm test (294 passed), npm run build
• cargo fmt --check, cargo clippy --all-targets -D warnings, cargo test (746 passed)
• New tests:
  • scan_items: split sizes for lengths 0 to 60, row→group→item traversal, Back to groups and Back to rows, reverse direction, no splitting in linear or plain grouped scanning, highlight surviving replace_rows.
  • scan_preferences: saving, reading and inheritance, and older-version and unknown-value compatibility.
  • portable_settings:
    • Version 2 upgrade, rejecting a version 2 document that already has rowGroups, and rejecting a version 3 document that lacks it.
    • A version 2 merge base still loads.
    • The synced scanPreferences key set is pinned.
  • settings_sync: updating from version 1 or 2 uploads nothing until something changes.
  • scan_keyboard: QWERTY statuses and highlighted tile counts.
  • PointScan.test.tsx: the three-way choice, per-area override, Use default, and card summaries.
• Not yet tried with a real switch on Windows or macOS.

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