feat(ui): failure panel

[sepehr-safari]

Summary

Surfaces the engine's detected failures in a failure panel — a fixed-height
drawer beneath the timeline that lists what detectFailures found for the active
trace, with one-click navigation to the evidence.

• Ranked list — one row per Failure, ordered critical → warning → info,
  then by first affected event. Each row shows a severity dot, the failure code,
  and its description.
• Expand for remediation — selecting a row expands its suggested_steps and
  affected event ids (accordion: at most one open at a time).
• Jump to event — selecting a failure also selects its primary event, so the
  timeline highlights it and the detail pane (feat(ui): message inspector + session panel #30) shows it.
• Clean + skipped states — a clean trace shows a positive "no failures
  detected"; a detection-skipped large trace (ADR-0007) says so. The status bar
  now carries the severity breakdown (e.g. 2 failures: 1 critical, 1 warning).

Design notes

• Layout. ui.split is horizontal-only (it lays panes along x), so the
  drawer isn't a vertical split — activeBody is a column: the timeline|detail
  split grows to fill, the 240px failure drawer sits below it. The drawer's list
  scrolls and is capped for pathological inputs.
• Ordering is display-only. sortedFailureIndices returns indices into the
  engine's failures slice; the slice itself is never reordered. Failure
  identity (the accordion's expanded_failure, each row's jump target) is the
  raw index, so sorting can't desync the state.
• Same jump semantics as feat(ui): message inspector + session panel #30. Selecting a failure selects + highlights its
  primary event. The literal timeline viewport scroll needs the runner eject
  tracked in feat(ui): interactive open — native dialog + drag-drop (ejected runner) #33 (the runtime owns the virtual list's offset); realistic failure
  evidence is usually already on screen.

Testing

native test -Dplatform=null — 82 headless tests green (macOS local; CI runs
both OSes). New coverage:

• Pure helpers: severity ordering (critical → warning → info, then first event)
  and the severity-breakdown summary string.
• Over the failed-auth conformance fixture: the FAILED_AUTHORIZATION row
  renders, remediation is hidden until expanded then shows its steps + affected
  events, and clicking the row expands it and selects the primary event.
• Workspace: select_failure toggles the accordion and jumps; out-of-range is
  ignored.
• The accessibility sweep now also covers the failure drawer with an expanded
  entry.

native check --strict clean; native build (ReleaseFast) builds. Living docs
(CURRENT_STATE.md) updated in-branch.

Closes #31