feat(ui): live-capture view — streaming timeline, controls, status

[sepehr-safari]

Part 2 of #59 — the interactive live-capture surface, on top of the pt1 streaming model (update_fx + LiveCapture, ADR-0009 / #67). This is the last issue in the S5 flagship before notifications (#60).

What's here

A new Surface toggle (traces | live) puts a live-capture surface beside the opened-trace workspace, reached from an always-present "Live" tab (and a button on the empty state). It renders through the same timeline / detail / failure widgets as an opened trace — driven off model.live.trace, which the model re-derives from the streamed NDJSON — so a live session correlates, detects, and stays viewport-bounded for free. Selection/toggle mutations (select_event, select_failure, payload-tree collapse, …) route to whichever surface is shown via Model.mutTrace, so one set of timeline widgets drives both.

The control strip (captureControls):

• Two endpoint fields — listen (host:port) and upstream (ws:// URL) — seeded with example defaults and edited in place (caret tracked model-side, mirroring the search field). Read-only while capturing (the session is bound to them for its life).
• A Start / Stop button — Start (primary) is gated on both endpoints being set; Stop (destructive) while running.
• A status line: a colored dot + label (Ready / Capturing… / Stopped / Failed (reason)) and an N events · M KB recorded readout.
• Empty / error states show a state-appropriate note; the live status bar mirrors the trace one over the live trace.

The filter bar stays a traces-surface affordance (the live surface always shows the full stream).

Tests (161/161, +7)

View tests drive the surface through the real dispatch path (msgForPointer / capture_line Msgs, exactly as the runtime + worker produce them):

• Start/Stop toggles the control and status; streamed events append to the live timeline and the status bar counts them.
• A streamed Invalid Authorize response lights up FAILED_AUTHORIZATION in the live failure panel — the failure list updates as the stream arrives.
• The live timeline stays under the widget budget (< 1024 nodes) at 500 streamed events.
• The live surface passes the a11y sweep (idle controls + capturing timeline/failure/detail).
• "Live capture" from the empty state switches surfaces.

Model tests cover surface routing (live selection leaves the workspace put) and endpoint editing + freeze-while-capturing.

Verification

• native test -Dplatform=null → 161/161.
• native build (ReleaseFast) → clean — the live view compiles into the app, not just tests (lazy-analysis check).
• zig fmt --check clean; studio ci still exits 0; the GUI launches without a startup crash (the Linux Xvfb smoke is the CI gate).

Not here (follow-up)

• feat(capture): OS notifications on critical failures #60 — OS notifications on critical live failures (services.showNotification + a notifications permission), the last S5 issue.

Closes #59.