feat(ui): live-capture streaming model + update_fx adoption (#59 pt1)

[sepehr-safari]

Part 1 of #59 (live-capture view): the effects-channel adoption and the streaming state machine from ADR-0009. Interactive view controls + a config form are part 2.

What's here

• update_fx adoption (main.zig). The app switches from the pure update to the effects-capable update_fx: it runs workspace.update for the model change, then issues fx.spawn on start_capture — self-spawning studio capture … --ndjson, whose stdout NDJSON lines stream back as capture_line Msgs — and fx.cancel on stop_capture. main captures argv[0] as the self-exe for the spawn argv.
• LiveCapture state machine (workspace.zig). New Msgs (start_capture / stop_capture / capture_line: EffectLine / capture_exit: EffectExit); the captured NDJSON stream accumulates (bounded to max_live_bytes — oldest whole lines drop), and the live trace is re-derived from it via loadTrace — reusing the whole engine, so it correlates + detects exactly like an opened trace and the inspector renders it viewport-bounded. Kept off the traces array so tab compaction never disturbs it.

Why re-derive (not incremental append)

It reuses the entire parse → correlate → detect → render pipeline for free (the live trace is a LoadedTrace), so a live session behaves identically to an opened one. Re-parse is O(n) per line — fine at human event rates within the cap; an incremental-append optimization is a later refinement.

Tests (154/154, +2)

Streamed lines append into a growing, correlated trace; status transitions across start / stop / exit reasons (.cancelled/.exited → stopped, .rejected/.spawn_failed/.signaled → failed); lines gate when not capturing; a fresh start bumps the cancel key and clears the recording. All leak-tested (std.testing.allocator).

Verification

• native test -Dplatform=null → 154/154.
• native build (ReleaseFast) → clean — the CLI dispatch already pulled run() into the app build, and now updateFx/fx.spawn/Effects.lineMsg compile there too.
• zig fmt --check clean; studio ci still exits 0 (no regression from the update_fx switch).

Part 2 (next)

The interactive view: a start/stop control + status, a config input (listen / upstream), and presenting the live trace (the model + render path are ready; this wires the buttons and the config). Then #60 (notifications).