feat(changeset): shared batch→changes resolver in core

[behinddwalls]

Summary

Add submitqueue/core/changeset, the single place the orchestrator resolves batch identity into the changes a batch contains — consolidating the batch -> requests -> changes walk that the build, merge, and score controllers each performed privately.

Resolver exposes two single-batch fidelities, both keyed per batch so callers with several batches loop and keep the per-batch boundary: ChangesForBatch returns raw changes (URIs only, no change-store read) for the build and merge stages, and DetailedForBatch returns one ChangeInfo per claimed URI with provider details read from the change store, for the score stage and detail-aware analyzers.

Ships with a store-backed implementation (depending only on the request and change stores), a programmable in-memory fake, a generated mock, and tests. The package is added unused; extensions adopt it in later branches. entity.BatchChanges is repurposed as DetailedForBatch's output (doc comment only). The mocks make-target gains the new package.

Test Plan

Issues

Stack

1. docs(rfc): extension contract — identity in, resolve internally #214
2. @ feat(changeset): shared batch→changes resolver in core #216
3. refactor(mergechecker): accept entity.Request, resolve change internally #217
4. refactor(changeprovider): accept entity.Request, resolve change internally #218
5. refactor(scorer): score entity.Batch, resolve changes internally #219
6. refactor(buildrunner): trigger on batches, resolve changes internally #221
7. refactor(pusher): push ordered batches, return per-batch outcomes #222
8. feat(conflict): target-overlap analyzer using changeset resolver #223
9. refactor(entity): relocate analyzer/checker/pusher result types to entity #227