feat(ui): live capture view — streaming timeline

[sepehr-safari]

Scope (gated on #58 / ADR-0009)

A live capture surface in the inspector — a timeline that appends events as
capture streams, with start/stop controls and live failure surfacing — reusing
the S3 virtualized timeline and failure panels so a long live session stays
viewport-bounded. Events arrive via the channel ADR-0009 settles (a self-spawned
studio capture --ndjson worker, or the runner-eject path).

Contingency: if ADR-0009 finds no in-runner channel (#58 outcome ii),
this issue's GUI portion moves to the #33 runner-eject bucket; the headless
capture flagship (#57) is unaffected.

Deliverables (assuming a channel)

• A live-capture view/mode in src/ui/: start/stop capture, connection state,
  a streaming timeline that appends canonical events, a live failure list, and
  a "recording to <path>" indicator.
• Back-pressure / bounds: cap retained live events (ring/window) so an unbounded
  session can't exhaust memory; the on-disk recording is the full record.

Acceptance criteria

• Headless view tests drive the live model: injected events append to the
  timeline; start/stop toggles state; the failure list updates; the view stays
  viewport-bounded under a large synthetic live stream.
• Both native build and native test green.

Dependencies

#58 (channel / ADR outcome), #56 (event source). Builds on S3 (#27–#32).

[sepehr-safari]

Spike #58 (ADR-0009) resolved → this proceeds in-runner; no #33 eject. The zero-config runtime has an effects channel: switch the app to update_fx: fn(*Model, Msg, *Effects) and, on "start capture", fx.spawn the app's own binary as studio capture --listen … --upstream … --ndjson in .lines mode. Each NDJSON line arrives as an on_line Msg → decode (capture.decode/parser) → append to the live timeline; fx.cancel(key) stops it. Crash-isolated subprocess, reusing the #57 CLI engine. Start with a minimal fx.spawn smoke (trivial command → assert on_line/on_exit) and resolve the self-exe path (argv[0] / selfExePath) for the spawn argv.

[sepehr-safari]

Part 1 landed (PR #67, merged): the update_fx adoption + the LiveCapture streaming state machine (ADR-0009). The app now runs on the effects channel (Xvfb smoke confirms the GUI opens with update_fx); fx.spawn/fx.cancel are wired; the model streams NDJSON capture_line Msgs into a re-derived live trace that correlates + detects like any opened trace. Headless-tested (append, status transitions, bounds).

Remaining (part 2) — the interactive view:

• A start/stop capture control + live status indicator.
• A config input for --listen / --upstream (currently set via configureCapture).
• Present model.live.trace (reuse the inspector timeline/failure panels) while capturing.

The model + engine render path are ready; part 2 wires the buttons and the config into inspector.zig.