Resolve unchecked MINOR daily-review findings (batch)

[Luis85]

Convert unchecked MINOR findings from 2026-04-26/27/28/30 daily reviews into code fixes with tests where applicable.

Checklist:

• CognitionPipeline.ts:143 — stage-blocked failure branch test coverage (2026-04-26)
• TfjsReasoner.ts:401 — concurrency safety in detectBestBackend (2026-04-26)
• PlayView.vue:11 — seed key duplication drift risk (2026-04-27)
• cognitionSwitcher.ts:442 — reasoner/learner wiring race window (2026-04-27)
• useAgentSession.ts:321-322 — stale-epoch dispose + learning-mode learner wiring (2026-04-28)
• useAgentSession.test.ts:415 — brittle microtask flush depth in tests (2026-04-30)

Keep NIT-only findings optional unless they gate maintainability.