feat(keyboard): leader-sequence runtime in the keymap dispatcher (RIG-2709)

[rigel-mintaka]

This PR is part of a stack containing 4 PRs:

1. main
2. feat(keyboard): sequence-grammar chord helpers for leader chords (RIG-2707) #612
3. feat(keyboard): four G-leader sequence rows + display/aria hardening (RIG-2708) #613
4. "feat(keyboard): leader-sequence runtime in the keymap dispatcher (RIG-2709)" (this PR)
5. docs(keyboard): document the leader-sequence runtime in the dispatch header (RIG-2710) #619

Arm-and-complete "press G, then " sequences inside the one keydown
handler installKeymap already owns — no second listener, no module global.

• Per-install pending leader state + a disarm() that clears the live
  timeout; leader set is leaderPrefixes(DEFAULT_KEYMAP, platform), computed
  once at install (table-derived, never hard-coded).
• Per-keydown order (§A3): normalize → editable-guard FIRST (modifier-less
  keys only) → completion (pure-modifier keys ignored, Escape disarms+consumes,
  else disarm then resolve the "<leader> <chord>" sequence rows; a dead
  sequence falls through and re-enters arming) → arming (leader prefix,
  no command modifier, not event.repeat; preventDefault+stopPropagation) →
  single-chord path (unchanged).
• Tier resolution factored into a resolve(matching, event) closure so a
  completed sequence and a single chord run byte-identical (active group →
  scoped → global, scope gate intact).
• isEditableTarget extended to HTMLSelectElement and
  [role=combobox|listbox|menu] so a bare leader key never steals native
  <select> typeahead or an ARIA widget's keys.
• Uninstaller clears any live disarm timer (no leak against a torn-down
  closure on test teardown / HMR).
• LEADER_TIMEOUT_MS = 1000 (OQ1 ratified) exported for runtime + tests.

Also folds the two deferred RIG-2707 review lows into leaderPrefixes's unit
coverage (first runtime caller): per-platform resolution of a Mod leader
segment, and distinct-leader accumulation with same-leader dedup.

Tests: dispatch.test.ts + keyboard-e2e.test.tsx (51 pass) cover arm/complete,
timeout disarm, editable + <select> + ARIA-widget guards, dead-sequence
fall-through, re-arm, Escape, lone-Shift-does-not-disarm, Mod+B-mid-sequence
disarm-and-resolve, held-key no-arm, and uninstall-clears-timer; the two e2e
cases drive G S/G L through real App wiring. keymap.test.ts 20 pass.

Ledger-impact: none (decisions landed with the design PR #544; DL-248..252).

RIG-2709

[linear-code]

RIG-2709

[github-actions]

Compass engineering docs preview: https://compass-ux-rig-2709-leader-r.compass-eng-docs.pages.dev

Deployed from compass-ux/rig-2709-leader-runtime at 761ad0f.