FSP wire format revision and session-layer MMP implementation

FSP wire format revision (TASK-2026-0007):

Introduce the FIPS Session Protocol (FSP) wire format with a 4-byte
common prefix [ver_phase:1][flags:1][payload_len:2 LE] replacing the
old 1-byte msg_type dispatch. All session messages share this prefix
with phase-based dispatch (Established, Setup, Ack, Unencrypted).

- New session_wire.rs: FSP constants, header types, parse/build helpers
- SessionMessageType enum: DataPacket (0x10), SenderReport (0x11),
  ReceiverReport (0x12), PathMtuNotification (0x13)
- FspFlags (CP/K/U) and FspInnerFlags (SP) for flag management
- SessionSenderReport, SessionReceiverReport, PathMtuNotification
  message structs with encode/decode
- FSP send pipeline: 12-byte header as AAD, 6-byte inner header
  (timestamp + msg_type + inner_flags), encrypt_with_aad()
- FSP receive pipeline: parse header, extract cleartext coords (CP),
  AEAD decrypt with AAD, strip inner header, msg_type dispatch
- Forwarding: transit nodes parse cleartext coords without decryption
- Removed DataPacket struct and associated types
- SessionEntry: session_start_ms, mark_established(), session_timestamp()
- FIPS_OVERHEAD: 144 → 150 bytes (+6 for FSP inner header)
- Design docs updated for new wire format

Session-layer MMP implementation (TASK-2026-0008):

Implement complete session-layer MMP reusing the link-layer algorithm
modules (SenderState, ReceiverState, MmpMetrics, SpinBitState) with
independent configuration and higher report interval clamps.

- SessionMmpConfig: separate config section (node.session_mmp.*)
- MmpSessionState: session-specific wrapper with PathMtuState tracking
- Session-layer constants (500ms-10s report intervals, 1s cold start)
- Parameterized interval methods (new_with_cold_start,
  update_report_interval_with_bounds) on SenderState/ReceiverState
- Bidirectional From conversions between link/session report types
- SessionEntry: mmp and is_initiator fields, initialized on Established
- send_session_msg() for reports/notifications
- Per-message RX recording with spin bit state tracking
- Handlers for SenderReport, ReceiverReport, PathMtuNotification
- path_mtu threaded from SessionDatagram envelope through to handlers
- check_session_mmp_reports() tick handler with collect-then-send pattern
- Periodic and teardown operator logging for session metrics
- PathMtuState: destination observes incoming MTU on all session messages,
  source seeded from outbound transport MTU, decrease-immediate /
  increase-requires-3-consecutive rules

Link-layer MMP fix:

- Stop feeding spin bit RTT samples into SRTT estimator; inter-frame
  timing in the mesh is irregular, inflating spin-bit RTT by variable
  processing delays; timestamp-echo provides accurate RTT

29 files changed, 602 tests pass, 0 clippy warnings.

src/protocol/link.rs

@@ -272,9 +272,9 @@ impl Disconnect {
 /// | 20     | dest_addr | 16 bytes | Destination node_addr               |
 /// | 36     | payload   | variable | Session-layer message               |
 ///
-/// The payload is either end-to-end encrypted (SessionSetup, SessionAck,
-/// DataPacket) or plaintext link-layer error signals (CoordsRequired,
-/// PathBroken) generated by transit routers.
+/// The payload is either end-to-end encrypted (handshake messages, data,
+/// reports) or plaintext error signals (CoordsRequired, PathBroken)
+/// generated by transit routers.
 #[derive(Clone, Debug)]
 pub struct SessionDatagram {
     /// Source node address (originator of this datagram).
@@ -526,7 +526,7 @@ mod tests {
     fn test_session_datagram_encode_decode() {
         let src = make_node_addr(0xAA);
         let dest = make_node_addr(0xBB);
-        let payload = vec![0x10, 0x00, 0x05, 0x00, 1, 2, 3, 4, 5]; // DataPacket payload
+        let payload = vec![0x10, 0x00, 0x05, 0x00, 1, 2, 3, 4, 5]; // session payload
         let dg = SessionDatagram::new(src, dest, payload.clone())
             .with_ttl(32);
 

src/protocol/mod.rs

@@ -37,10 +37,12 @@ pub use tree::TreeAnnounce;
 pub use filter::FilterAnnounce;
 pub use discovery::{LookupRequest, LookupResponse};
 pub use session::{
-    CoordsRequired, DataFlags, DataPacket, PathBroken, SessionAck, SessionFlags,
-    SessionMessageType, SessionSetup, COORDS_REQUIRED_SIZE, DATA_FLAG_COORDS_PRESENT,
-    DATA_HEADER_SIZE,
+    CoordsRequired, FspFlags, FspInnerFlags, PathBroken, PathMtuNotification, SessionAck,
+    SessionFlags, SessionMessageType, SessionReceiverReport, SessionSenderReport, SessionSetup,
+    COORDS_REQUIRED_SIZE, PATH_MTU_NOTIFICATION_SIZE, SESSION_RECEIVER_REPORT_SIZE,
+    SESSION_SENDER_REPORT_SIZE,
 };
+pub(crate) use session::{decode_optional_coords, encode_coords};
 
 /// Protocol version for message compatibility.
 pub const PROTOCOL_VERSION: u8 = 1;