feat(speculation): standard composed speculator

[behinddwalls]

Summary

Why?

The Generator ranks and the Allocator spends; something must compose them into the Speculator extension the orchestrator calls.

What?

Adds speculator/standard. It funds the queue's most promising paths first until the build budget is spent: candidates considered in descending likelihood, in-flight paths kept funded rather than restarted, finished paths never re-proposed. Pairing bestfirst with sticky yields the default speculation policy; changing either behavior means swapping a part, not writing a new Speculator.

standard adds no cancellation handling of its own and inherits it from the two parts; a test pins that a cancelled run yields the context error and no actions. Behavior is documented in standard/README.md; the speculator README keeps only the extension contract.

Test Plan

✅ bazel test //submitqueue/extension/speculation/...

✅ make fmt, make gazelle, make mocks

Stack

1. docs(rfc): explain best-first speculation generation #513
2. feat(speculation): generator contract and bestfirst impl #446
3. feat(speculation): allocator contract and sticky impl #450
4. @ feat(speculation): standard composed speculator #451
5. feat(storage): speculation path set store #501
6. feat(storage): path-build link store for per-path builds #502
7. feat(orchestrator): dispatch builds per speculation path #503
8. feat(orchestrator): poll builds and stop the ones nothing wants #504
9. fix(orchestrator): mint distinct message IDs for cancel re-publishes #505
10. feat(orchestrator): re-plan the queue from the Speculator each run #506
11. feat(orchestrator)!: finalize batches from their speculation paths #507
12. feat(orchestrator): compose per-queue speculators and turn speculation on #508