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/upper/icmp.rs

@@ -62,13 +62,21 @@ const MAX_ORIGINAL_PACKET: usize = MIN_IPV6_MTU - IPV6_HEADER_LEN - ICMPV6_HEADE
 /// Total FIPS encapsulation overhead (worst case).
 ///
 /// Breakdown:
-/// - Noise encryption tag: 16 bytes
-/// - Established frame header: 16 bytes (common prefix + receiver_idx + counter)
-/// - Inner header: 5 bytes (4-byte timestamp + 1-byte msg_type)
+/// - Noise encryption tag: 16 bytes (FLP link-layer AEAD)
+/// - FLP established frame header: 16 bytes (common prefix + receiver_idx + counter)
+/// - FLP inner header: 5 bytes (4-byte timestamp + 1-byte msg_type)
 /// - SessionDatagram fields: 35 bytes (ttl + path_mtu + src_addr + dest_addr)
-/// - DataPacket header: 12 bytes (msg_type + flags + counter + payload_len)
+/// - FSP header: 12 bytes (4-byte prefix + 8-byte counter)
+/// - FSP inner header: 6 bytes (4-byte timestamp + 1-byte msg_type + 1-byte inner_flags)
+/// - Session AEAD tag: 16 bytes (FSP session-layer AEAD)
 /// - Coordinates (worst case): ~60 bytes (2 coords with depth 3 each)
-pub const FIPS_OVERHEAD: u16 = 16 + 16 + 5 + 35 + 12 + 60; // 144 bytes
+///
+/// The old 12-byte session header is replaced by FSP header (12) + FSP inner
+/// header (6) + session AEAD tag (16) = 34 bytes, an increase of 22 bytes.
+/// However, the old overhead already accounted for the session AEAD tag in
+/// the "Noise encryption tag" line, and the old header included the counter.
+/// Net change: +6 bytes for FSP inner header.
+pub const FIPS_OVERHEAD: u16 = 16 + 16 + 5 + 35 + 12 + 6 + 60; // 150 bytes
 
 /// Calculate the effective IPv6 MTU for FIPS-encapsulated traffic.
 ///