feat(speculation): add prioritizer + prioritization-limit extensions

[behinddwalls]

Add the queue-wide prioritizer seam and its limit counterpart from the speculation RFC, as vendor-agnostic extension interfaces under submitqueue/extension/speculation/.

prioritizer: selection is per batch and blind to other batches, so it cannot ration a shared build budget. The prioritizer sees every path across the queue's in-flight batches that runs or wants to run (Selected / Prioritized / Building, each carrying its ID and score), ranks them — scores are probabilities in [0, 1] per the path scorer's contract — and returns sparse decisions naming paths by ID: Promote to admit under the budget, Cancel to preempt, at most one decision per path. Whether it preempts at all is implementation policy (sticky-slots vs preemptive). It never writes: the controller maps each decision back to its tree and applies the status transition under that tree's optimistic lock. Prioritized thus means exactly "admitted under the queue's build budget, cleared to build, not yet building".

prioritizationlimit: the "how much" policy bounding the queue's concurrent speculation builds — the budget the prioritizer admits against. Injected into the prioritizer at construction and applied by it, keeping the interface limit-free; the value is signal-driven, not a fixed constant.

Each follows the repo extension contract: Factory.For(Config) (T, error) with Config carrying only QueueName. Includes READMEs, gomock packages, and programmable fakes. Interfaces only; concrete impls and controller wiring are deferred.

Stack

1. feat(entity): give speculation paths an assigned identity #337
2. feat(speculation): add enumerator + dependency-limit extensions #315
3. feat(speculation): add path scorer extension #316
4. feat(speculation): add selector + selection-limit extensions #317
5. @ feat(speculation): add prioritizer + prioritization-limit extensions #320
6. feat(storage,entity): path identity + speculation path→build mapping store #331
7. feat(speculation): add parity default impls for the speculation seams #332
8. feat(speculation): add probability path scorer #333