feat(speculation): generator contract and bestfirst impl

[behinddwalls]

Summary

Why?

The standard Speculator needs a ranked stream of candidate speculation paths to fund. Nothing enumerates or ranks the path space today.

What?

Adds submitqueue/extension/speculation/generator — the Generator/PathIterator contract — plus the bestfirst implementation and mocks.

The generator is a pure enumerator: Open takes only the queue's batches and offers every coherent path, including ones whose builds already ran. Suppressing those belongs to the Allocator, which already reconciles candidates against the stored path sets by ID.

bestfirst ranks each path by how likely all its assumptions are to hold and generates lazily: one heap holds the frontier of every head's path tree, so pulling k paths does O(k) work however large the space behind them — which is also why the old depth bound is gone. Scores are summed log probabilities so wide heads cannot underflow into ties, and a canonical subset expansion emits each path exactly once, in non-increasing score order. The full design — tie-breaks, per-dependency derivation, the floating-point pitfalls — is documented in generator/bestfirst/README.md and next to the code.

Also sharpens scorer.Scorer's contract: it returns the probability that a batch's build succeeds — the quantity bestfirst consumes — not "the likelihood of a successful land".

Test Plan

✅ bazel test //submitqueue/extension/... — ordering, uniqueness, canonical score sums, wide-head underflow (7100-dependency heads), score validation, context cancellation, and the no-copy Open contract.

✅ make fmt, make gazelle, make mocks

Issues

Stack

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