feat(ui): search & filter — close out S3

[sepehr-safari]

Summary

Make large traces navigable: search & filter the event timeline by structured
facets and free text, instantly, and close out S3.

Scope

• Filter facets — action, direction (CS→CSMS / CSMS→CS / unknown), message
  type (Call / CallResult / CallError), severity (participates in a failure of
  level X), and session / transaction id. Model-owned, composable (AND across
  facets).
• Free-text search — a search-field (built-in clear affordance) matching
  action / unique id / payload substring, case-insensitive.
• Instant — the timeline's virtual-list source becomes the filtered index set;
  filtering stays viewport-sized in node budget and fast on large traces (derive
  the filtered index list in the build arena — no per-event widgets for hidden
  rows). Result count shows in the status bar.
• Clear — clearing search/filters restores the full timeline; empty result
  shows a quiet "no matching events" state.

Acceptance criteria

• Each facet and free-text search filters the timeline correctly and
  composably; the result count is accurate.
• Filtering a large trace keeps widget nodes viewport-sized (no materialization
  of hidden rows) — headless node-count assertion.
• search-field clear (x / Escape) restores results through the input path.
• Headless tests over the filter/search predicate + the filtered-index derive.
• native test -Dplatform=null green on macOS + Linux; native check --strict clean; CURRENT_STATE/AGENTS/ROADMAP updated in-PR; S3 closed.

Dependencies

Milestone S3. Depends on #28 (the timeline it filters). Closes out the
milestone.