FLP wire format revision and MMP link-layer measurement protocol

## FLP Wire Format Revision

Replace the 1-byte discriminator with a structured wire format:

- 4-byte common prefix (ver+phase, flags, payload_len) and 16-byte
  established frame header with AEAD AAD binding
- 5-byte encrypted inner header (4-byte session-relative timestamp +
  1-byte message type) on all link messages
- Phase-based packet dispatch replacing discriminator-based dispatch
- SessionDatagram reassigned from type 0x40 to 0x00; add SenderReport
  (0x01) and ReceiverReport (0x02) message types for MMP
- SessionDatagram: rename hop_limit to ttl, add path_mtu field (u16 LE)
  with min(datagram.path_mtu, transport.mtu()) at forwarding
- Updated handshake packets (msg1: 87->90 bytes, msg2: 42->45 bytes)
- FIPS_OVERHEAD updated from 135 to 144 bytes

## MMP Link-Layer Measurement Protocol

Add the Metrics Measurement Protocol for link quality measurement
between FIPS peers. Measures RTT, loss, jitter, throughput, OWD trend,
and ETX from periodic sender/receiver reports exchanged over established
links.

Module layout:
- mmp/algorithms.rs: JitterEstimator, SrttEstimator, DualEwma, OwdTrend,
  SpinBit, ETX computation
- mmp/report.rs: SenderReport (48B) and ReceiverReport (68B) wire format
- mmp/sender.rs: per-peer TX counters and interval tracking
- mmp/receiver.rs: per-peer RX counters, jitter, loss, gap tracking
- mmp/metrics.rs: derived metrics from report processing (SRTT, goodput_bps)
- mmp/mod.rs: MmpMode (Full/Lightweight/Minimal), MmpConfig, MmpPeerState
- node/handlers/mmp.rs: report dispatch, timer-driven generation, operator
  logging (periodic + teardown)

Integration: per-frame TX/RX hooks in encrypted message handling, report
dispatch from link message router, timer-driven generation from tick
handler, and periodic operator logging with throughput formatting.

Three operating modes: Full (sender + receiver reports, spin bit, CE echo),
Lightweight (receiver reports only), Minimal (spin bit + CE echo only).

## Design Documentation

Updated FLP sections across all design documents to match the implemented
wire format, including revised overhead calculations and numeric values.

568 tests pass, clippy clean.

src/config/node.rs

@@ -7,6 +7,7 @@
 use serde::{Deserialize, Serialize};
 
 use super::IdentityConfig;
+use crate::mmp::MmpConfig;
 
 // ============================================================================
 // Node Configuration Subsections
@@ -213,9 +214,9 @@ impl BloomConfig {
 /// Session/data plane (`node.session.*`).
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct SessionConfig {
-    /// Default SessionDatagram hop limit (`node.session.default_hop_limit`).
-    #[serde(default = "SessionConfig::default_hop_limit")]
-    pub default_hop_limit: u8,
+    /// Default SessionDatagram TTL (`node.session.default_ttl`).
+    #[serde(default = "SessionConfig::default_ttl")]
+    pub default_ttl: u8,
     /// Queue depth per dest during session establishment (`node.session.pending_packets_per_dest`).
     #[serde(default = "SessionConfig::default_pending_packets_per_dest")]
     pub pending_packets_per_dest: usize,
@@ -236,7 +237,7 @@ pub struct SessionConfig {
 impl Default for SessionConfig {
     fn default() -> Self {
         Self {
-            default_hop_limit: 64,
+            default_ttl: 64,
             pending_packets_per_dest: 16,
             pending_max_destinations: 256,
             idle_timeout_secs: 90,
@@ -246,7 +247,7 @@ impl Default for SessionConfig {
 }
 
 impl SessionConfig {
-    fn default_hop_limit() -> u8 { 64 }
+    fn default_ttl() -> u8 { 64 }
     fn default_pending_packets_per_dest() -> usize { 16 }
     fn default_pending_max_destinations() -> usize { 256 }
     fn default_idle_timeout_secs() -> u64 { 90 }
@@ -341,6 +342,10 @@ pub struct NodeConfig {
     /// Internal buffers (`node.buffers.*`).
     #[serde(default)]
     pub buffers: BuffersConfig,
+
+    /// Metrics Measurement Protocol (`node.mmp.*`).
+    #[serde(default)]
+    pub mmp: MmpConfig,
 }
 
 impl Default for NodeConfig {
@@ -359,6 +364,7 @@ impl Default for NodeConfig {
             bloom: BloomConfig::default(),
             session: SessionConfig::default(),
             buffers: BuffersConfig::default(),
+            mmp: MmpConfig::default(),
         }
     }
 }