feat(ui): inspector shell — workspace model + trace loading

[sepehr-safari]

Summary

The foundation of the S3 native inspector — the placeholder counter is gone and
the inspector's shell, workspace state, and trace-loading path are in.

• Zig builder view, not markup (ADR-0006). The main view is a hand-written
  canvas.Ui builder view (src/ui/inspector.zig), because the event timeline
  (next issue) needs the builder-only windowed virtual list to stay within the
  1024 widget-node budget at dataset scale. src/app.native is removed.
• Bounded multi-trace workspace (src/ui/workspace.zig) — the Model / Msg
  / update half of the loop. Each open trace owns a private engine arena
  (parse → timeline → detection) that is freed on close; the model allocates
  nothing per rebuild.
• Trace loading from the command line — studio path/to/trace.json … reads
  each file in main through init.io (unbounded — the large-trace path builds
  on this) and seeds the workspace. A built-in embedded sample backs the empty
  state's "Open sample" and the smoke test.
• The shell — empty state, a workspace tab strip when more than one trace is
  open, a per-trace overview (event / session / failure / warning counts), an
  error state for unreadable or malformed traces, and a status bar. The window
  grows to inspector size (1200×800, min 900×560).

Verification

• native test -Dplatform=null — 61/61 pass (macOS; CI also runs Linux).
  New coverage: workspace transitions (open / select / close / bounds / malformed,
  leak-checked with the testing allocator), the view built through the real
  canvas.Ui tree and driven via msgForPointer, the error state, an
  accessibility sweep (empty + loaded), and engine layout.
• native check --strict clean; native build produces the ReleaseFast binary.
• scripts/smoke.sh updated to launch with the sample trace and assert the
  loaded overview — the CI Linux · Xvfb job drives the real window.

Scope notes

• Interactive open (native dialog + drag-drop) is intentionally not
  here — the zero-config UiApp exposes no view/effect path to showOpenDialog
  or file-drop routing, so it needs an ejected runner. Tracked in feat(ui): interactive open — native dialog + drag-drop (ejected runner) #33 with an
  eject ADR, per the S3 planning decision.
• The event timeline, message inspector, failure panel, and search/filter are the
  following S3 issues (feat(ui): virtualized event timeline #28–feat(ui): search & filter — close out S3 #32).

Closes #27