feat(speculation): standard composed speculator

[behinddwalls]

Summary

Why?

The three branches below this one define the pieces: the Speculator contract (#445), a Generator that ranks candidate paths (#446), and an Allocator that spends the build budget on them (#450). This one joins them up.

Keeping the composition in its own implementation means a queue can change how paths are scored, or how the budget is rationed, independently — and without either choice reaching the Speculator contract the controller depends on.

What?

Adds submitqueue/extension/speculation/speculator/standard.

Speculate opens the generator over the queue's batches and path sets, then hands the resulting candidate iterator to the allocator. That is the whole implementation: it makes no decision of its own, so its behavior is entirely the behavior of the two parts it is built from. Pairing bestfirst with sticky gives the default policy — rank paths by how likely their guesses are to hold, fill the free budget in that order, never preempt a running build.

Also adds a note to the speculator package README.md describing the composition.

Test Plan

✅ bazel test //submitqueue/extension/speculation/... (3 targets), make lint, make check-gazelle

One end-to-end test wires the real bestfirst and sticky together and checks the pair proposes only builds, and that the funded path for a dependent batch is the optimistic guess. Two mock-based tests cover the wiring itself: that the iterator the generator returns is the one handed to the allocator, and that a generator error propagates rather than being swallowed.

Note the commit also updates a sticky.New call site for the logger argument added in #450 — the signature change landed there, and this is the only caller.

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
4. feat(speculation): allocator contract and sticky impl #450
5. @ feat(speculation): standard composed speculator #451

[sbalabanov]

move to "standard"'s README
The description should say not how it is composed, but what it does (what behavior it implements wrt prioritizing paths and working with build budget).