feat(p7-06): port memstore & store interface tests (38 tests)
Add JetStreamMemoryStoreTests (27 tests, T:2023-2056) and StorageEngineTests (11 tests, T:2943-2957) covering the JetStream memory store and IStreamStore interface. Fix 10 bugs in MemStore.cs discovered during test authoring: FirstSeq constructor, Truncate(0) SubjectTree reset, PurgeEx subject-filtered implementation, UpdateConfig MaxMsgsPer enforcement, FilteredStateLocked partial range scan, StoreRawMsgLocked DiscardNewPer, MultiLastSeqs maxSeq fallback scan + LastNeedsUpdate recalculation, AllLastSeqs LastNeedsUpdate recalculation, LoadLastLocked LazySubjectState recalculation, GetSeqFromTime ts==last equality, and timestamp precision (100-ns throughout). 20 tests deferred (internal fields, benchmarks, TTL, filestore-only). All 701 unit tests pass.
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