feat(keyboard): four G-leader sequence rows + display/aria hardening (RIG-2708)

[rigel-mintaka]

This PR is part of a stack containing 3 PRs:

1. main
2. "feat(keyboard): four G-leader sequence rows + display/aria hardening (RIG-2708)" (this PR)
3. feat(keyboard): leader-sequence runtime in the keymap dispatcher (RIG-2709) #616
4. docs(keyboard): document the leader-sequence runtime in the dispatch header (RIG-2710) #619

Adds the four Go-to leader sequences from the frozen leader-chords design (docs/designs/product/compass-leader-chords/design.md, T2 / §A4-A5) to DEFAULT_KEYMAP and hardens the two shipped chord scanners so a sequence renders correctly everywhere and never reaches aria-keyshortcuts.

• DEFAULT_KEYMAP: a new // Go-to sequences (RIG-2484) block with G B → view.bridge, G L → view.backlog, G D → view.done, G S → view.settings, each placed after any existing modifier row for the same command so the modifier chord stays the first match.
• shortcutForAria now skips sequence rows (chordSegments(entry.chord).length === 1): a sequence-only command returns undefined (its writer omits the attribute) and a dual-bound command returns its modifier chord. A press-then-press sequence has no valid WAI-ARIA representation, so the skip is mandatory and makes the result row-order-independent.
• shortcutFor now returns formatChordForDisplay of the first matching row and does NOT skip sequences, so view.backlog/view.done display "G then L"/"G then D" and keep their point-of-use chip and title; a dual-bound command still shows its modifier chord because that row is first.
• The ? overlay join formats via formatChordForDisplay, so a sequence row reads "G then B" instead of the misleading "G B".

Tests: keymap.test.ts moves view.backlog out of the shortcutFor miss group, adds a sequence-only-display case, a dual-bound modifier-wins case, and a DEFAULT_KEYMAP authoring-invariant block (every sequence is two modifier-less segments; no leader prefix is also a complete single chord); ShortcutsOverlay.test.tsx updates the bridge-filter row count 1→2 and asserts the formatted "G then B"; Palette.test.tsx asserts Backlog/Done carry no aria-keyshortcuts but a "Backlog (G then L)"/"Done (G then D)" title while Bridge keeps aria-keyshortcuts="Control+B".

Stacked on RIG-2707 (the sequence-grammar helpers). The dispatcher leader runtime that makes these chords fire is T3 (RIG-2709); until it lands the rows are advertised only through the discoverability net, which is why T2 and T3 ship in one stack.

Ledger-impact: none. DL-248..DL-252 for this record landed with the design PR (#544).

Refs RIG-2708

Co-authored-by: Matt Wilkinson matt@rigel.build

[linear-code]

RIG-2708

[github-actions]

Compass engineering docs preview: https://compass-ux-rig-2708-leader-s.compass-eng-docs.pages.dev

Deployed from compass-ux/rig-2708-leader-sequence-rows at 5c93e2e.