Expand fips-intro.md: prior work, MMP section, review fixes

- Expand Prior Work from 4 entries to 11 subsections covering STP,
  Yggdrasil/Ironwood, split-horizon, cryptographic identity (CJDNS,
  Tor, HIP), dual-layer encryption, Noise IK/XK/IKpsk2, index-based
  dispatch, transport-agnostic mesh, MMP measurement precedents
  (RTCP, Jacobson SRTT, QUIC spin bit, ETX, ECN), and Nostr primitives
- Add Metrics Measurement Protocol (MMP) section between routing and
  transport abstraction
- Fix Lightning Network Noise pattern: XK not IK
- Qualify transport observer claims (traffic patterns visible, FIPS
  identities not extractable from ciphertext)
- Add NAT traversal gap acknowledgment in transport section
- Standardize fd00::/8 notation (was fd::/8)
- Replace ambiguous "FIPS address" with explicit pubkey/node_addr/IPv6
  distinction
- Align identity section privacy qualifier with security section
- Separate Kleinberg and Thorup-Zwick attributions
- Merge redundant Protocol Architecture / Architecture Overview sections
- Add Sybil/zero-config tradeoff, eclipse attack, traffic analysis
  out-of-scope notes to security section
- Add key rotation tradeoff note to identity section
- Add bloom filter sizing future-analysis note
- Update External References with all new citations

src/lib.rs

@@ -1,4 +1,4 @@
-//! FIPS: Federated Interoperable Peering System
+//! FIPS: Free Internetworking Peering System
 //!
 //! A distributed, decentralized network routing protocol for mesh nodes
 //! connecting over arbitrary transports.