feat(speculation): add probability path scorer

[behinddwalls]

Summary

Why?

The pathscorer seam has no real implementation — only mocks and a programmable fake — yet its output is the ranking currency the selector and prioritizer act on. Speculation needs a default scorer whose scores mean something from the start: inert while trees hold a single path, but ranking chain-vs-fallback paths correctly the moment an enumerator produces alternatives. It is split out from the parity-stub impls because it is the one seam implementation with an actual model, worth reviewing on its own.

What?

pathscorer/probability — scores each path as the probability that exactly that path's assumption holds: Score = p(head) × Π p(d) for base deps × Π (1−p(d)) for non-base head deps.

Each batch's probability resolves from its state, so resolved outcomes override predictions: succeeded → 1, failed/cancelled → 0, otherwise the score stage's predicted Batch.Score (neutral 0.5 when unscored; best-effort cancelling keeps its prediction). A dead dependency therefore zeroes every path built on it and boosts every path that excluded it by the full (1−p)=1 factor — score mass shifts to the paths consistent with reality, with no cross-path coupling. Folding outcomes into path status stays the controller's reconcile job; the scorer only recomputes Score, over every path regardless of status.

Dependencies are read one at a time through the injected storage.BatchStore per the store's key/value contract, each batch loaded at most once per call; store errors return wrapped and unclassified. The README walks a three-path worked example (chain / drop-B / alone) before and after a dependency failure.

Test Plan

✅ make gazelle && make fmt && bazel test //submitqueue/extension/speculation/.... Unit coverage: probability-formula table including all state-resolution cases (succeeded/failed/cancelled in and out of base, cancelling keeps prediction, unscored fallbacks), a three-path dependency-failure scenario pinning the score-mass shift, per-dep single store read, error propagation, empty-tree short-circuit.

Stack

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