feat(store)!: extract nexus-wake + nexus-inmemory — tokio out of nexus-store (#300)

[joeldsouzax]

Why

nexus-store's production code is no_std+alloc-clean except tokio, confined to notify.rs behind the subscription feature — a packaging choice, not a design dependency (#224 already cut the WakeSource seam). This is the prerequisite for the store no_std port (#301) and moves the public path nexus_store::notify::* before the 1.0 freeze makes it permanent.

What

• nexus-wake — StreamNotifiers/WakeReg/NotifyError moved verbatim (git mv); owns tokio/foldhash/parking_lot. The StreamNotifiers white-box tests from wake.rs moved along.
• nexus-inmemory — InMemoryStore + InMemorySnapshotStore as a first-class adapter crate (RawEventStore + WakeSource + SnapshotStore + StreamLister/AtomicAppend behind export/import). StoredFrame now carries its SchemaVersion typed (captured at append like offsets), so the read path never re-parses header bytes — wire::SCHEMA_VERSION_OFFSET stays pub(crate) and the unreachable CorruptSchemaVersion variant is gone.
• nexus-store — zero tokio/foldhash/parking_lot; subscription is a dep-free gate; the testing feature is deleted.
• fjall/postgres repoint nexus_store::notify → nexus_wake; examples repoint nexus_store::testing → nexus_inmemory.

Deviation from the issue text

#300 kept InMemoryStore in nexus-store with a dev-dep on nexus-wake — but InMemoryStore embeds StreamNotifiers structurally behind a feature, and features cannot enable dev-dependencies, so that design is a normal-dep cycle cargo rejects. Extracting the adapter crate (as discussed) resolves it.

Test relocation

A dev-dep cycle unifies types only for integration tests; the lib-test target recompiles the crate under cfg(test) as a distinct crate, so nexus-inmemory's impls satisfy none of its bounds. Hence:

• The InMemoryStore-fixture mods of export/import/cbor/decoded/projection moved to tests/ (import's private plan_section tests stayed inline; cbor's white-box mod stayed, only the store-using pipeline test moved).
• inmemory_store_tests + inmemory_conformance moved into nexus-inmemory.
• The catchup/subscription_cursor white-box mods (pub(crate) seams: CATCHUP_CHUNK, live_stepped) drive an in-crate TestStore double (src/test_support.rs, cfg(test)-only, ~200 lines); the public subscribe surface stays proven against the real adapter + real nexus-wake in tests/subscription_tests.rs.

Acceptance (from #300)

• nexus-store has no tokio/foldhash/parking_lot dependency
• nexus-fjall + nexus-postgres build against nexus-wake
• nix flake check green (pre-commit); 1051/1051 workspace tests pass; clippy clean under --all-features --all-targets

Closes #300

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