feat(speculation): add selector + selection-limit extensions

[behinddwalls]

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

selector: the controller hands it the batch's speculation tree; it returns per-path decisions (Promote/Cancel), each naming a path by its ID (entity.SpeculationPathInfo.ID), for the paths it chooses to act on — at most one decision per path, with conflicting duplicates treated by the controller as a policy bug (applied first, logged and skipped after). It is the policy seam — it reads the tree it is given and emits decisions only, never writing status, and does not decide merging. The controller maps each decision to a guarded status transition (Promote → Selected, Cancel → Cancelling/Cancelled) and persists it, staying the single writer of tree state. entity.PathDecision correspondingly names paths by PathID rather than restating the Base/Head split.

selectionlimit: the "how much" policy bounding how many paths a batch may build in parallel. It is the selector's companion — the selector decides which paths, the limit decides how many. Injected into the selector at construction and called by it, keeping the selector interface limit-free; the value is dynamic, 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