Add startup process-exit diagnostics

[Ustice]

Summary

• Add a helper-owned startup-process-exit diagnostics bundle for timberborn_process_exit failures.
• Capture before/after Timberborn error-report inventory, changed error-report copies, launch intent metadata, exact process snapshot, Player.log copy, bounded Player.log scan/tail, and citeable summary paths.
• Preserve the Stabilize guarded Timberborn startup when Steam stays frontmost #76 guarded startup launch semantics: no retry loop, no manual focus/open fallback, and no duplicate open_bundle path.

Closes #78.

Verification

• bun test tests/timberborn-startup.test.ts passed: 15 tests.
• bun run typecheck passed.
• git diff --check a5296192..ed43d39f passed.
• No Timberborn launch attempted.

Review

• Reviewer PASS recorded on Investigate guarded Timberborn startup exit after Steam-frontmost recovery #78: Investigate guarded Timberborn startup exit after Steam-frontmost recovery #78 (comment)
• Coordinator fallback Tech-Lead review performed after the Tech-Lead thread stalled; Claude CLI fallback was unavailable due invalid auth. Diff scope, helper boundary, launch semantics, bounded/citeable artifacts, and deterministic checks were verified locally.