Design documentation illustration pass and FLP→FMP rename

Rename FIPS Link Protocol (FLP) to FIPS Mesh Protocol (FMP)

  The "Link Protocol" name understated the layer's scope — spanning tree
  construction, bloom filter routing, greedy forwarding, and mesh-wide
  coordination go well beyond link-level concerns. Rename fips-link-layer.md
  to fips-mesh-layer.md, update FLP→FMP throughout docs and source code
  (FLP_VERSION→FMP_VERSION, wire.rs, rx_loop.rs, spanning_tree.rs).

New SVG illustrations

  - Protocol stack: color-coded layer diagram replacing ASCII art
  - OSI mapping: side-by-side comparison with traditional networking layers
  - Bloom filter propagation: 6-node tree with sender-colored filter boxes
    showing split-horizon computation per link
  - Routing decision flowchart: 5-step priority chain with candidate ranking
    by tree distance and link performance
  - Coordinate discovery: sequence diagram showing LookupRequest propagation,
    response caching, and SessionSetup cache warming

Redesigned existing SVGs

  - Architecture overview: uniform node layout, U-shaped encrypted link
    connectors, separate end-to-end session line
  - Node architecture: split Router Core into FSP and FMP layers, reorganize
    transports into Overlay/Shared Medium/Point-to-Point categories
  - Identity derivation: wider boxes, visible encode arrow, dashed npub line

fips-intro.md revisions

  - Add inline references to prior work: Yggdrasil/Ironwood for coordinate
    routing, Noise Protocol Framework for IK handshakes, WireGuard for
    index-based session dispatch, Wikipedia for bloom filters, split-horizon,
    and greedy embedding
  - Add explanatory paragraphs after bloom filter diagram describing
    split-horizon filter computation and candidate selection behavior
  - Simplify transport abstraction language, remove I2P/LoRa references
  - Fix LookupRequest wording ("propagates" not "floods"), note intermediate
    node coordinate caching on lookup responses
  - Rewrite architecture overview prose to match redesigned diagrams

src/upper/icmp.rs

@@ -69,22 +69,22 @@ const MAX_ORIGINAL_PACKET: usize = MIN_IPV6_MTU - IPV6_HEADER_LEN - ICMPV6_HEADE
 /// Breakdown (traced through the actual send path):
 ///
 /// ```text
-/// FLP outer header (cleartext AAD)              16
+/// FMP outer header (cleartext AAD)              16
 ///   common prefix (4) + receiver_idx (4) + counter (8)
-/// FLP AEAD ciphertext:
-///   timestamp (4) + msg_type (1)                 5   [FLP inner header]
+/// FMP AEAD ciphertext:
+///   timestamp (4) + msg_type (1)                 5   [FMP inner header]
 ///   ttl (1) + path_mtu (2) + src (16) + dst (16) 35  [SessionDatagram body]
 ///   FSP header (4 prefix + 8 counter)            12   [cleartext AAD]
 ///   FSP AEAD ciphertext:
 ///     timestamp (4) + msg_type (1) + flags (1)    6   [FSP inner header]
 ///     <application data>
 ///     Poly1305 tag                               16   [FSP AEAD]
-/// FLP Poly1305 tag                              16   [FLP AEAD]
+/// FMP Poly1305 tag                              16   [FMP AEAD]
 ///                                              ────
 ///                                               106
 /// ```
 ///
-/// Note: the FLP inner header msg_type byte IS the SessionDatagram msg_type
+/// Note: the FMP inner header msg_type byte IS the SessionDatagram msg_type
 /// byte (shared, not double-counted). The "35 bytes" is the SessionDatagram
 /// body after msg_type is consumed by the dispatch layer.
 pub const FIPS_OVERHEAD: u16 = 16 + 16 + 5 + 35 + 12 + 6 + 16; // 106 bytes