mr-668: add GraphId newtype + Cloud-mode forward identity stubs (PR 1/10)

PR 1 of the MR-668 multi-graph server work. Pure types, no runtime
behavior changes yet.

Ships the validated identity vocabulary that the rest of the implementation
will consume:

- `GraphId(String)` — `^[a-zA-Z0-9-]{1,64}$`, leading underscore rejected
  (engine reserves every `_*` filename), reserved route names rejected
  (`policies`, `healthz`, `openapi`, `openapi.json`, `graphs`). Validation
  lives in `try_from` only; serde `Deserialize` re-runs it so JSON payloads
  cannot bypass.
- `TenantId(String)` — same regex shape as GraphId. `None` in Cluster
  mode; reserved for Cloud mode (RFC 0003) where it carries the OAuth
  `org_id` claim.
- `GraphKey { tenant_id: Option<TenantId>, graph_id }` — the registry
  HashMap key. `cluster()` constructor for the Cluster-mode default.
- `Scope` enum with `Full` variant — Cluster mode default; RFC 0004 will
  extend with OAuth scopes (`graph:read`/`write`/`admin`/`*`).
- `AuthSource` enum with `Static` variant — Cluster mode default; RFC
  0001 step 1 will add `Oidc`.
- `ResolvedActor { actor_id, tenant_id, scopes, source }` — replaces the
  upcoming refactor of `AuthenticatedActor(Arc<str>)` in PR 4a.

Per MR-668 design decision 13: ship the Cloud-mode forward type shapes
now (no `TokenVerifier` trait yet — that's RFC 0001 step 1) so handler
signatures stay stable across the Cluster → Cloud trajectory. `Scope`
and `AuthSource` use `#[non_exhaustive]` so future variants don't break
caller matches.

Tests: 26 new (15 graph_id + 11 identity), all passing. No regression
in the existing 36 server library tests.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/omnigraph-server/src/lib.rs

@@ -1,9 +1,14 @@
 pub mod api;
 pub mod auth;
 pub mod config;
+pub mod graph_id;
+pub mod identity;
 pub mod policy;
 pub mod workload;
 
+pub use graph_id::GraphId;
+pub use identity::{AuthSource, GraphKey, ResolvedActor, Scope, TenantId};
+
 use std::collections::{HashMap, HashSet};
 use std::fs;
 use std::io;