feat(storage): path-build link store for per-path builds - #502

## Summary

### Why?

Speculation needs one build per *path*, and something must record which build belongs to which path attempt. The runner chooses the build ID, so a caller holding a path cannot derive it; a keyed record is the key-value contract's mechanism for a reverse lookup.

### What?

`entity.PathBuild` is new: the `(pathID, attempt) -> buildID` link, with `storage.PathBuildStore`, a MySQL implementation, schema, and mocks.

The record is write-once: created only once the runner has named the build, never updated. Absent means no build is recorded for the attempt; present names its build permanently, and a retried path is a new attempt under a different key. Because creation is the only write, the first insert also decides concurrent dispatches for the same attempt — `ErrAlreadyExists` tells the loser the attempt's build is someone else's. There is deliberately no reservation state and no version column: the stage that watches builds (later in this stack) stops any build whose path no longer wants it, so no reader needs to distinguish "idle" from "mid-dispatch".

`entity.Build` gains `PathID` and `Attempt`, and `entity.SpeculationPath` gains `Base()` — the dependencies the path assumes will succeed — so every consumer derives the base one way.

## Test Plan

✅ `bazel test //submitqueue/entity/... //submitqueue/extension/storage/...`

✅ `bazel test //test/integration/submitqueue/extension/storage/mysql:go_default_test` — an attempt resolving to its build, `ErrNotFound` when undispatched, and a duplicate `Create` refused so the first link stands.

✅ `make fmt`, `make gazelle`, `make mocks`

# Conflicts:
#	submitqueue/extension/storage/BUILD.bazel
#	submitqueue/extension/storage/mock/BUILD.bazel
#	submitqueue/extension/storage/mock/storage_mock.go
#	submitqueue/extension/storage/mysql/BUILD.bazel
#	submitqueue/extension/storage/mysql/build_store.go
#	submitqueue/extension/storage/mysql/build_store_test.go
#	submitqueue/extension/storage/mysql/storage.go
#	submitqueue/extension/storage/storage.go

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# Changes to be committed:
#	modified:   submitqueue/entity/BUILD.bazel
#	modified:   submitqueue/entity/build.go
#	new file:   submitqueue/entity/path_build.go
#	modified:   submitqueue/entity/speculation.go
#	modified:   submitqueue/extension/storage/BUILD.bazel
#	modified:   submitqueue/extension/storage/mock/BUILD.bazel
#	new file:   submitqueue/extension/storage/mock/path_build_store_mock.go
#	modified:   submitqueue/extension/storage/mock/storage_mock.go
#	modified:   submitqueue/extension/storage/mysql/BUILD.bazel
#	modified:   submitqueue/extension/storage/mysql/build_store.go
#	modified:   submitqueue/extension/storage/mysql/build_store_test.go
#	new file:   submitqueue/extension/storage/mysql/path_build_store.go
#	modified:   submitqueue/extension/storage/mysql/schema/build.sql
#	new file:   submitqueue/extension/storage/mysql/schema/path_build.sql
#	modified:   submitqueue/extension/storage/mysql/storage.go
#	new file:   submitqueue/extension/storage/path_build_store.go
#	modified:   submitqueue/extension/storage/storage.go
#	modified:   test/integration/submitqueue/extension/storage/suite.go
#