fix(runtime): make startupInfoEmitted atomic (#3593)

[aheritier]

Summary

Fixes #3593 — startupInfoEmitted was a plain bool with a check-then-set
pattern, raced by App.Start emitting startup info from a spawned goroutine
(pkg/app/app.go) while reEmitStartupInfo resets and re-emits from another
(e.g. on a new session). Result: a data race plus duplicate or missed startup
info.

This is finding A4 from the internal codebase audit.

What changed

• startupInfoEmitted is now atomic.Bool. EmitStartupInfo uses
  CompareAndSwap(false, true) so only the CAS winner emits within an epoch;
  ResetStartupInfo uses Store(false) to open a new epoch allowing exactly
  one subsequent re-emit.

Testing

• pkg/runtime/startupinfo_race_test.go — concurrent start + re-emit, plus a
  single-emission invariant. Verified to fail (data race + duplicate/missed
  emission) against the pre-fix plain-bool code and pass now.
• task dev + go test -race -count=2 ./pkg/runtime/... ./pkg/app/...: green
  (validated in isolation).

Merge note

Minor overlap with #3587 (A6) on the LocalRuntime struct: A6 changes
alignment / adjacent fields while keeping the old bool. Reconciliation keeps
A6's fields and this branch's atomic.Bool. Not a blocker in isolation.

Opened as a draft pending final maintainer review.