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 a per-path decision (Promote/Cancel) for the paths it chooses to act on. 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.

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,storage): rework speculation tree model and store #231
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