fuzzy-match + recency-rank the command palette

Completes the command-palette acceptance: the filter is now a
subsequence fuzzy match (was substring), and recently-invoked commands
float to the top and break score ties. The subsequence matcher is
extracted to a shared fuzzy helper so the palette and quick-open file
finder use one implementation instead of a private copy each.

Pinned commands and cross-session recency persistence are left as a
follow-up (they need a pin affordance + settings storage).

Co-Authored-By: Claude <noreply@anthropic.com>

lib/kernel/src/fuzzy.dart

@@ -0,0 +1,27 @@
+/// Subsequence fuzzy matching, shared by the command palette and the
+/// quick-open file finder. Pure Dart (no Flutter) so it's reusable across
+/// isolates and trivially unit-testable.
+library;
+
+/// Returns null when [query]'s characters don't appear in order within
+/// [text]; otherwise a score where **lower is better** — contiguous, early
+/// matches score best; gaps between matched chars and a late first match add
+/// penalty. Callers that want case-insensitive matching should lowercase both
+/// arguments first.
+int? fuzzyScore(String text, String query) {
+  if (query.isEmpty) return 0;
+  var ti = 0;
+  var qi = 0;
+  var score = 0;
+  int? last;
+  while (ti < text.length && qi < query.length) {
+    if (text.codeUnitAt(ti) == query.codeUnitAt(qi)) {
+      score += last == null ? ti : (ti - last - 1);
+      last = ti;
+      qi++;
+    }
+    ti++;
+  }
+  if (qi != query.length) return null;
+  return score;
+}