feat: knowledge-graph resource + observe identity provenance

[rrader26]

Rust parity with @memmesh/sdk v0.9.0 — see
thinkfleet-memory-sdk#21.

New: mm.graph()

Method	Endpoint
stats()	GET /admin/memory/graph/stats
list_entities(..)	GET /admin/memory/entities
get_entity(id, ..)	GET /admin/memory/entities/:id
list_edges(..)	GET /admin/memory/graph/edges
traverse(id, ..)	POST /admin/memory/graph/traverse

There was no graph surface before, so the structural half of memory — the
entities and typed edges extraction builds — was unreachable from Rust.

Use stats(), not list_entities(..).len() for any "how big is it"
question. The list routes page, so their length is the page size. Against a real
project that is the difference between reporting 1,000 and the actual 12,142.

Edges are hydrated, and modelling them wrong is a hard failure

list_edges, traverse, and get_entity().edges return the server's
GraphTraversalEdge — subject and object are full entities, not ids,
plus a hop counter. There is no subjectId / objectId / brainId on the
wire.

I found this by running the port against production, where strict decoding blew
up with missing field 'subjectId'. The same mistake was live in the TypeScript
PR, where structural typing hid it — the call compiled and handed back objects
whose declared fields were all undefined at runtime. Fixed in both.

The raw memory_edge row shape is deliberately not modelled: no read route
returns it, and a type nothing returns is a trap.

hop is 0 from list_edges (no seed) and 1-indexed from traverse.

Query encoding

Filter values are percent-encoded. An unescaped & in a search string would
otherwise truncate the query server-side and quietly return the wrong page —
covered by with_query_encodes_values.

observe forwards identity

user_id / agent_id / session_id now reach the server. The route has always
accepted them; the SDK was dropping them, so provenance never arrived.
Inserted only when set, so an existing call site sends an identical request
(observe_text_omits_identity_when_unset).

Provenance, not a tenancy boundary — search filters
chatIdentityId IS NULL OR = $1, permissive by design.

Verification

• 113 tests pass (9 new), cargo clippy --all-targets clean.
• Verified live against app.memmesh.ai via examples/graph_live.rs:
  12,142 entities / 287,698 edges, decoding real edges as
  NVIDIA CORP -[reported_metric]-> Cost of Revenue and the literal-object case
  NVIDIA CORP -[ticker_symbol]-> NVDA.

Remaining parity gaps

.NET and Go are next. Go is furthest behind — no raw-text observe at all.