feat(speculation): add best-first speculation path generator

[sbalabanov]

Summary

Implements the Generator seam of the standard Speculator (see
doc/rfc/submitqueue/speculation.md): a pull-based stream of candidate
speculation paths over one queue snapshot, consumed by the Allocator.

• generator: the Generator/PathIterator contract — lazy, snapshot-scoped,
  ctx-aborted; candidates never repeat and never contradict resolved facts.
• generator/bestfirst: prices every path by the probability that all of its
  dependency assumptions hold — resolved deps pinned as facts,
  Merging/Cancelling priced as modal certainties, undecided deps scored via
  the injected scorer (once per batch per run) — and yields candidates in
  exact non-increasing price order through lazy add/shift enumeration merged
  across heads by one max-heap. Dependencies are normalized into canonical
  queue order so path IDs stay stable across runs; ties are deterministic.
• doc/rfc/submitqueue/speculation-generator.md: design doc with the pricing
  model, enumeration scheme, a step-by-step two-run worked example, and
  future refinements (relaxation, unblocking weight, sensitivity pruning).

Also regenerates storage/mock/request_batch_store_mock.go, whose committed
header predated generation via 'make mocks' and kept check-mocks red.

Co-Authored-By: Claude Fable 5 noreply@anthropic.com

Test Plan

Issues

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}