feat: M2 + M3 — AuthService, MLS group lifecycle, Delivery Service

M2:
- schemas/auth.capnp: AuthenticationService (upload/fetch KeyPackage)
- noiseml-core: IdentityKeypair (Ed25519), generate_key_package, NoiseTransport
  with send_envelope/recv_envelope, Noise_XX handshake (initiator + responder)
- noiseml-proto: auth_capnp module, ParsedEnvelope helpers
- noiseml-server: AuthServiceImpl backed by DashMap queue (single-use KPs)
- noiseml-client: register + fetch-key subcommands, ping over Noise_XX
- tests: auth_service integration test (upload → fetch round-trip)

M3:
- schemas/delivery.capnp: DeliveryService (enqueue/fetch opaque payloads)
- noiseml-core/group.rs: GroupMember — MLS group lifecycle
  create_group, add_member (→ Commit+Welcome), join_group, send_message,
  receive_message; uses openmls 0.5 public API (extract() not into_welcome,
  KeyPackageIn::validate() not From<KeyPackageIn>)
- noiseml-server: DeliveryServiceImpl on port 7001 alongside AS on 7000
- noiseml-proto: delivery_capnp module

TODO (see M3_STATUS.md):
- noiseml-client: group subcommands (create-group, invite, join, send, recv)
- noiseml-client/tests/mls_group.rs: full MLS round-trip integration test

crates/noiseml-proto/build.rs

@@ -31,12 +31,22 @@ fn main() {
         "cargo:rerun-if-changed={}",
         schemas_dir.join("envelope.capnp").display()
     );
+    println!(
+        "cargo:rerun-if-changed={}",
+        schemas_dir.join("auth.capnp").display()
+    );
+    println!(
+        "cargo:rerun-if-changed={}",
+        schemas_dir.join("delivery.capnp").display()
+    );
 
     capnpc::CompilerCommand::new()
         // Treat `schemas/` as the include root so that inter-schema imports
-        // (e.g. `using import "/auth.capnp"`) resolve correctly in later milestones.
+        // resolve correctly.
         .src_prefix(&schemas_dir)
         .file(schemas_dir.join("envelope.capnp"))
+        .file(schemas_dir.join("auth.capnp"))
+        .file(schemas_dir.join("delivery.capnp"))
         .run()
         .expect(
             "Cap'n Proto schema compilation failed. \