feat(speculation): add path scorer extension

[behinddwalls]

Add the scorer seam from the speculation RFC, as a vendor-agnostic extension interface under submitqueue/extension/speculation/scorer/.

The scorer computes each speculation path's predicted-success score from the current state: the per-batch scores of the path's base batches (entity.Batch.Score) and which of those dependencies have resolved (landed or build-passed), plus optionally other signals. It is a prediction over live state, so the controller re-runs it on every respeculate right after reconciling status, and persists the result; the scorer owns only the formula.

The store is the source of truth: the scorer takes only the batch identity and loads what it needs — the batch's speculation tree (with controller-reconciled statuses) and its dependency batches — through read access injected at its Factory. It returns the scored tree; the controller persists the scores, staying the single writer of tree state.

This is the per-path scorer, distinct from the existing per-batch score stage (extension/scorer) that sets entity.Batch.Score — the path scorer consumes those to score whole paths.

Follows the repo extension contract: Factory.For(Config) (Scorer, error) with Config carrying only QueueName. Includes README, gomock package, and a programmable fake. Interface 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(prioritization): add prioritizer + prioritization-limit extensions #318