feat: add combobox filter

docs/CHECKLIST.md

@@ -19,9 +19,6 @@ Gaps surfaced by the signed comparison; ordered by signed's usage.
 - [ ] **Editable code field** — syntax-highlighted editable multiline field,
       the code view's highlighter over the input's editing model; signed's
       repo file browser.
-- [ ] **Searchable list filter** — searchable collection behind combobox and
-      list option sources; the combobox's pluggable filter covers part of it
-      (signed's `SearchableVec`).
 - [ ] **Dropdown menu** — trigger button + popover + menu composite, three
       signed call sites; must stay a real component, not a wrapper.
 - [ ] **Motion easing helpers** — cubic ease-out and lerp beside
@@ -71,7 +68,8 @@ Gaps surfaced by the signed comparison; ordered by signed's usage.
       settings-row variants
 - [x] Status dot — colored state bead, Badge's variant vocabulary, optional
       screen-reader label
-- [x] Combobox — type-to-filter picker, create / keep / revert blur modes
+- [x] Combobox — type-to-filter picker with a caller-owned matcher, create /
+      keep / revert blur modes
 - [x] Command palette — filtered actions; the matcher stays caller-owned
 - [x] Scrollbar — overlay thumbs for gpui's `list` and `uniform_list`,
       platform autohide with a `visibility` override