Session-scoped buffers (implements ADR-0014)

[tetsuh]

Summary

Add a <prefix>/buffers/<sid>/<key> scope: a session-scoped, disk-backed,
pull+push large-value store. One put fans the full payload out to live
subscribers and stores identical bytes for later get. Persistence is per-session
(kDurable / kEphemeral).

References

• ADR-0014 (session-scoped buffers) — normative
• docs/02_architecture.md §2 (key space), §4 (StorageNode)
• docs/03_wire_protocol.md §1 (key space)
• Related: ADR-0002, ADR-0003

Scope

1. key.hpp: add KeyKind::Buffer; BuildBufferKey(sid, key); ParseKey
   branch for buffers/<sid>/<key>; validation reuses existing <sid>/<key>
   grammar. No $batch, no snap counterpart.
2. Config / SessionOptions: BufferPersistence { kDurable, kEphemeral };
   SessionOptions{buffers} per session; Config global default (kDurable).
3. SessionController: on CreateSession, for kDurable create a per-session
   disk StorageEngine for buffers; on CloseSession, purge/close it (subsequent
   get returns not-found).
4. StorageNode: routing branch for buffers/**:
   • put → for kDurable, write bytes to the session buffer engine; zenoh fans
     the same bytes to live subscribers. For kEphemeral, no store.
   • get/List → for kDurable, respond from the session buffer engine; for
     kEphemeral, respond empty/not-found.
   • delete → remove from the session buffer engine.
5. Late-join: document and test the querying-subscriber ordering
   (get buffers/<sid>/**, then subscribe), mirroring ParamCache::Attach.
6. Docs: apply the patches in doc-patches.md to docs/02 §2 and
   docs/03 §1.

Acceptance criteria (AC)

• AC1: BuildBufferKey / ParseKey round-trip for valid keys; invalid <sid>
  or <key> rejected; buffers/** never parses as Base/Session/Snapshot.
• AC2: A put to buffers/<sid>/<key> in a kDurable session is (a) delivered
  in full to a pre-existing subscriber and (b) returned identically by a later
  get. Byte-for-byte equality asserted.
• AC3: A late-joining querying subscriber receives all values put before it
  attached, with no lost updates under concurrent puts.
• AC4: In a kEphemeral session, live subscribers receive the payload but get
  returns not-found and nothing is written to disk.
• AC5: After CloseSession, get on any buffers/<sid>/** key returns
  not-found; the per-session engine's storage is released.
• AC6: ParamCache subscribed to session/<sid>/** does not receive any
  buffers/<sid>/** payloads (scope isolation test).
• AC7: A raw zenoh client can put and get a buffer following docs/03 §1
  (interop test), consistent with existing RawZenohClient* tests.

Dependencies

• Depends on: StorageEngine abstraction (done), StorageNode, SessionController
  (v0.2), and a disk engine for kDurable = RocksDBEngine (v0.4, [M1] RocksDBEngine #8).
• Not required: snapshots, batch, ack.

Out of scope (follow-ups)

• Shared-memory zero-copy for same-host transfers.
• Per-subscriber reliability tuning and backpressure policy.
• HTTP GET access (covered by the gateway issue / ADR-0015).