feat(speculation): speculator extension contract

[behinddwalls]

Summary

Why?

Queue-scoped speculation (#413) puts one seam in front of the speculate controller. What the controller can delegate — which paths to build, which in-flight ones to preempt — goes through it; what it must not — whether a batch merges or fails — stays behind it.

A Speculator cannot express a verdict. A batch's outcome is computed by the controller, so swapping implementations changes which paths run, never whether a batch lands. That is enforced by the return type, not by convention.

What?

Adds submitqueue/extension/speculation/speculator.

• Speculate takes the queue's in-flight batches plus finalized ones still referenced as dependencies, and every path set for them — live and recently finished, so an implementation won't re-propose a path that already passed or failed. It returns the build and cancel actions it proposes; a path left as-is has no entry.
• Cancel is its only cancel power: preempting an in-flight path to free budget. Correctness cancels belong to the controller.
• Config (queue name) and Factory follow the per-queue extension pattern. Budget, depth bound, and clock are injected at construction, keeping the contract backend-agnostic.
• Adds the package README.md and generated mocks, and registers the package with the mocks Makefile target.

Contract only — no implementation, no wiring.

Test Plan

✅ bazel build //submitqueue/extension/speculation/...

No unit tests on this branch: it adds an interface, a config struct, and generated mocks, with no behavior to exercise. The contract is covered by the tests on the implementation branches.

Stack

1. feat(entity): speculation path and run entities #444
2. @ feat(speculation): speculator extension contract #445
3. feat(speculation): generator contract and bestfirst impl #446