refactor: unify Looper candidate-panel execution semantics

Summary

Extract a shared candidate-panel execution primitive for Looper-family algorithms so concurrency, ordering, timeout, quorum, cancellation, draining, accounting, and partial-failure behavior have one contract.

Parent: #2336 Related amplification budget: #1456 Related attempt trace: #2855

Current behavior

Fusion, ReMoM, Workflows, and Ratings each own a similar but different goroutine/semaphore/collector implementation:

• Fusion and Workflows actively cancel from their collectors when quorum is reached.
• ReMoM reaches quorum in its collector and cancels through the outer execution context on return.
• Ratings waits for every candidate and has no quorum or round timeout.
• Ordering, error policy, cancellation accounting, and late-result handling are algorithm-specific.

The duplication makes resource accounting and future Best-of-N/adaptive sampling harder to implement safely.

Scope

• Introduce a shared internal panel executor with explicit inputs for concurrency, timeout/deadline, quorum, ordering, and error policy.
• Define cancellation ownership and whether/how outstanding results are drained and accounted.
• Preserve deterministic candidate ordering independent of completion order.
• Return structured attempted/successful/failed/cancelled results suitable for #2855 and #1456.
• Migrate Fusion and ReMoM first, then Workflows and Ratings with behavior-preserving tests.

Non-goals

• Do not add Best-of-N, adaptive-N, semantic deduplication, or a new public algorithm.
• Do not change algorithm output bodies during the initial migration.
• Do not merge tool-trajectory state into the generic executor.

Acceptance criteria

• All migrated algorithms share one tested concurrency/quorum/cancellation primitive.
• Existing ordering and partial-failure behavior remain compatible unless an explicit bug is documented.
• Exact-quorum, over-quorum, timeout, fail-fast, skip, cancellation, and all-failed cases are tested.
• No goroutine or semaphore-slot leaks occur under cancellation.
• Attempt and usage accounting has a documented contract for cancelled/in-flight calls.
• Existing Looper microbenchmarks do not regress beyond repository thresholds.

Validation

• Targeted Looper unit tests under -race.
• Existing Fusion/ReMoM/Workflows/Ratings tests.
• Looper performance benchmark gate.