feat(storage): speculation path set store

[behinddwalls]

Summary

Why?

The speculate run reads every head's path set at the start of a run and persists what it funds at the end. entity.SpeculationPathSet exists, but nothing can store one.

What?

Adds storage.SpeculationPathSetStore, keyed on the head batch ID, with a MySQL implementation, schema, mocks, and contract tests.

The mutation is a conditional whole-item put — Update replaces the set, guarded on version — because a conditional put on a key is the primitive every backend offers directly; a field-level update is the one shape non-SQL backends would have to emulate with a read-modify-write. Version arguments are explicit and the entity's own Version field is ignored, per the storage README.

No secondary index: callers that want a queue's live sets enumerate the heads from the batch listing they already hold and read each set by key.

Test Plan

✅ bazel test //submitqueue/extension/storage/... — sqlmock coverage of get/create/update and every error contract, plus a pin that Update ignores the entity's Version field.

✅ bazel test //test/integration/submitqueue/extension/storage/mysql:go_default_test — against real MySQL: round-trip (each stored ID still hashes its stored path), missing head, duplicate create, and a compare-and-swap race where the loser must not restore the path the winner dropped.

✅ 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