fix(bb-app): bound startup restart failures

[MPIsaac-Per]

Human comments

What was wrong

The full-stack launcher retried failed child starts at a fixed interval without a terminal state. Diagnostic artifacts were unbounded and could remain empty when their filesystem writes failed. Startup storage errors before child supervision could also escape as a generic exit.

What changed

• Exponential, abortable restart backoff with an eight-failure terminal exit (75).
• Stable-lifecycle accounting based on the child exit time.
• Terminal stderr reporting for storage failures during startup, runtime-file setup, and cleanup.
• Atomic diagnostic artifacts, cleanup on failed rename, and a five-per-process-and-kind retention cap.

How you verified

pnpm exec turbo run test typecheck --filter=bb-app --filter=@bb/process-utils

• bb-app: 83 tests passed.
• @bb/process-utils: 22 tests passed.

Fixes #3340

AGENT GENERATED