Add FIFO Scenario scheduling

[Roman Lutz (romanlutz)]

Summary

Adds process-local single-active FIFO scheduling, stable queue positions, queued cancellation, race-safe handoff, overload evidence, and lightweight startup reconciliation. Reconciliation only owns scheduler-managed local runs and is non-destructive for shared backends.

What this layer adds

• One active Scenario run per backend process with stable FIFO ordering.
• Queue position, enqueue time, queued cancellation, and race-safe handoff.
• Scheduler provenance retained through Scenario metadata construction.
• Lightweight startup reconciliation scoped to scheduler-managed local runs.
• Structured target overload evidence without pretending to provide adaptive throttling.

Preview

Stack

This is 5 of 7 in native GitHub Stack #2390 and depends on #2375.

Layer	Scope	Pull request
1	Progress foundation	#2372
2	Catalog and launch	#2373
3	Live progress	#2374
4	Run history	#2375
5	FIFO scheduling (this PR)	#2376
6	Configuration sizing	#2377
7	Result details	#2378

See the full implementation, screenshots, walkthrough, validation, and operational notes.

Validation

• Backend: 15,212 passed, 120 skipped.
• Frontend: 68/68 suites, 1,286/1,286 tests; lint, TypeScript, and production build passed.
• Mock E2E, retries disabled: 91 passed, 0 failed, 0 skipped.
• Seeded E2E, retries disabled: 62 passed, 0 failed, 0 skipped.
• All configured pre-commit hooks passed; distinguished-engineer review approved the final stack.

Operational boundary

Scheduling and locks are process-local. Multi-worker or multi-replica deployments need durable coordination and leases; that is intentionally deferred and documented in the gist.