feat(task): emit_trial fans out to writer + relay; guaranteed teardown

[larsrollik]

PR A of the emit_trial convergence (docs/plans/PLAN_task_isolation_emit_trial.md §7). The framework-guarantee piece: make emit_trial the single per-trial seam and ensure teardown always runs, so no task can end a session with unpersisted data. Prompted by an audit that found the bare periodic_trigger protocol writes no .msw.df.jsonl at all because each task hand-rolls its own write + relay.

What

• TaskRunner.emit_trial(trial, *, relay=None) — one call now (a) durably writes the scored trial through the injected TrialDataWriter, and (b) if a relay= event is given (the task's flat monitor event, a different shape from the stored trial), dispatches it to the LogAgent relay queue best-effort (contextlib.suppress — a down/full monitor never interrupts the task). Migrated tasks stop hand-rolling put_nowait boilerplate; the write and relay can't drift apart.
• Guaranteed teardown — TaskProcess.__exit__ now calls exit_safely() from a finally, so the trial-data writer is flushed (close() when trial_count > 0) and hardware is torn down even if a post-hook or finalize_acquisition_in_session raises. Previously a raise there skipped teardown, so a session could end with its df unflushed or a device left open.

Compatibility

Fully additive: relay defaults to None (existing emit_trial(dict) callers unchanged), and un-migrated tasks that still write their own file are untouched. No task-boundary change. The finalize error still propagates — teardown is guaranteed to run, but a real finalize failure is not swallowed.

Tests

tests/test_emit_trial.py: relay dispatched only when relay= is passed; the full dict is written while the flat event is relayed; a raising relay queue is swallowed; and teardown (writer close() + clear) runs even when finalize_acquisition_in_session raises. Full msw-core suite green.

Next

PR B (msw-tasks-lab) migrates the trigger family onto this seam and fixes periodic_trigger; PR C migrates the remaining tasks; PR D adds the two-payload canonical record shared with control-plane §T8.