Robin/ct 977 doc subscriptions

[ubik2]

Creating draft PR for tracking

---

Summary by cubic

Lays groundwork for doc‑level subscriptions (CT‑977) and speeds up schema traversal. Reuses a single ContextualFlowControl and simplifies session types across memory and space.

• Performance

  • Reuse one ContextualFlowControl per traversal; pass through getAtPath, followPointer, narrowSchema to avoid repeated allocation.
  • selectSchema creates and reuses the same ContextualFlowControl (no behavior change expected).
  • Expanded traverse timing test and split large fixture into JSON; fixed path/value mismatch in test setup.

• Refactors

  • Consolidated session types: introduced SpaceStoreSession; internal Session is no longer exported; Space implements SpaceStoreSession; Memory maps spaces by DID.
  • Updated Memory internals (subscribe/unsubscribe/query/transact/mount/close) to operate on a minimal MemorySessionState.
  • Added design doc: docs/future-tasks/doc-level-subscriptions.md outlining cache-based doc-level subscription approach and tradeoffs.

^{Written for commit 0e07ae3. Summary will update automatically on new commits.}

[linear]

CT-977 Doc Level Subscriptions

[ubik2]

The shared cfc object landed separately in main, so it's not really part of this PR anymore