feat(grpc): scaffold tonic server for rustyred.v1.GraphDatabase (C3 Track 1)

Adds gRPC support to rustyred-server alongside the existing axum HTTP
routes. Single port, content-type sniffing routes application/grpc*
requests to the tonic service tree and everything else stays on the
existing axum handlers.

Files added:
  .gitmodules
  proto/                              (submodule → theorem-protos)
  crates/rustyred-server/build.rs     (tonic-build compiles rustyred.v1
                                      from the submodule path)
  crates/rustyred-server/src/grpc/    (new module)
    mod.rs                            (Routes builder + proto re-export)
    service.rs                        (GraphDatabaseService impl)

Deps added to rustyred-server:
  tonic = "0.12"
  prost = "0.13"
  tower = "0.5"
  async-trait = "0.1"
  tonic-build = "0.12"  (build-dep)

Implemented this commit (vital signs that prove the architecture):
  - Health     (returns ok + version)
  - Ready      (mirrors axum /ready handler via store_ready())
  - GraphStats (delegates to tenant_graph_store().stats())

PENDING (29 methods stubbed with Status::unimplemented):
  Each stub names the axum HTTP route it should mirror, so the
  fill-in work is bounded and mechanical. Categories: native query
  surface, transactions, node/edge primitives, bulk ingest, vector
  search, fulltext, spatial, epistemic traversal, algorithms (PPR,
  PageRank, components, communities), verify/rebuild, cache.

Mount pattern (main.rs):
  let http = router::build_router(state.clone());
  let grpc = grpc::build_grpc_routes(state).into_axum_router();
  let app = http.merge(grpc);
  axum::serve(listener, app).await

Gates:
  cargo build -p rustyred-server : clean (1 pre-existing dead-code warning)
  cargo test --workspace          : 222 passed, 0 failed

README updated to document the submodule init step.

Author: Travis-Gilbert

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "proto"]
+	path = proto
+	url = https://github.com/Travis-Gilbert/theorem-protos.git