add keymap multi-chord sequences and a stateful matcher

T-205, the resolver foundation for Vim motions (dd, gg, dw, ciw) and
repeat counts (5j). KeymapBinding now holds an ordered chord sequence
(length 1 for the common single-chord case); `keys:` parses a space-
separated spec into that sequence (D-82). Keymap.resolve keeps the
single-chord fast path; a new stateless Keymap.match answers
exact/prefix/none for a pending buffer.

SequenceMatcher wraps that query with a pending buffer, a repeat-count
prefix (leading digits, 0 excluded since it's the line-start motion),
the d-vs-dd timeout case (flush fires the buffered exact), and broken-
sequence recovery (discard, restart on the last chord). It is headless —
no keyboard reads, no event swallowing — so the editor (T-206) can drive
it from Focus.onKeyEvent and act on the result.

Also drops a stray unused import in the Vim indicator test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/kernel/src/keymap/keymap_service.dart

@@ -128,7 +128,7 @@ class KeymapService extends ChangeNotifier {
   /// settings overlay.
   void registerCommandBinding(String chordSpec, String commandId, {String? when}) {
     _contributions.add(KeymapBinding(
-      chord: KeyChord.parse(chordSpec),
+      sequence: KeyChord.parseSequence(chordSpec),
       intent: InvokeCommandIntent(commandId),
       when: WhenExpr.tryParse(when),
     ));