test(replay,integration): repo-knowledge replay derivation + state-keyed scan reuse

Adds two test files to prove B4/B3 invariants end-to-end:

- RepoKnowledgeReplayTest (testing/replay): projection-based replay tests that
  verify recorded RepoKnowledgeRetrievedEvent drives relevantFiles context assembly
  without consulting a retriever (test A), and that a log without the retrieval event
  falls back to the RepoMapComputedEvent legacy path (test B).

- RepoMapReuseIntegrationTest (testing/integration): end-to-end test using the real
  ProjectMemoryService + L3RepoKnowledgeRetriever + InMemoryL3MemoryStore proving
  exactly one index walk across two sessions sharing the same stateKey, and that the
  second session receives non-empty repo hits via the shared L3 embeddings despite
  having no per-session RepoMapComputedEvent.

New test deps (noted per constraint):
- testing/replay: adds core:context (needed to import ContextLayer/ContextEntry from
  the buildRelevantFilesEntry return type).
- testing/integration: adds apps:server (ProjectMemoryService, L3RepoKnowledgeRetriever,
  FakeWorkspaceStateProbe), core:router (InMemoryL3MemoryStore), core:config (ProjectConfig).
  No new inter-core or upward production dependencies introduced.

testing/replay/build.gradle

@@ -8,6 +8,7 @@ dependencies {
     testImplementation(project(":core:events"))
     testImplementation(project(":core:journal"))
     testImplementation(project(":core:kernel"))
+    testImplementation(project(":core:context"))
     testImplementation(project(":core:sessions"))
     testImplementation(project(":core:transitions"))
     testImplementation(project(":core:validation"))