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

docs/design/fips-session-layer.md

@@ -1,7 +1,7 @@
 # FIPS Session Protocol (FSP)
 
 The FIPS Session Protocol is the top protocol layer in the FIPS stack. It sits
-above the FIPS Link Protocol (FLP) and below applications (native FIPS API or
+above the FIPS Mesh Protocol (FMP) and below applications (native FIPS API or
 IPv6 adapter). FSP provides end-to-end authenticated, encrypted datagram
 delivery between any two FIPS nodes, regardless of how many intermediate hops
 separate them.
@@ -34,7 +34,7 @@ Applications access the FIPS mesh through two interfaces, both served by FSP:
 
 Applications address destinations directly by npub or public key. The FIPS
 stack resolves the destination's node_addr, establishes or reuses a session,
-encrypts the payload, and routes through FLP. No DNS involvement.
+encrypts the payload, and routes through FMP. No DNS involvement.
 
 ### IPv6 Adapter
 
@@ -66,21 +66,21 @@ layer.
   TCP over IPv6 get TCP's congestion control; native API applications must
   manage their own sending rate.
 
-## Services Required from FLP
+## Services Required from FMP
 
-FSP treats FLP as a black box providing three services. FSP knows nothing about
+FSP treats FMP as a black box providing three services. FSP knows nothing about
 transports, transport addresses, links, peers, spanning trees, coordinates,
 bloom filters, hop counts, or network topology.
 
 ### SessionDatagram Forwarding
 
-FLP accepts a SessionDatagram (source node_addr, destination node_addr, TTL,
+FMP accepts a SessionDatagram (source node_addr, destination node_addr, TTL,
 path MTU, payload) and delivers it best-effort toward the destination. Delivery
 may traverse multiple hops, each with independent link encryption.
 
 ### Error Signaling
 
-FLP signals routing failures asynchronously:
+FMP signals routing failures asynchronously:
 
 - **CoordsRequired**: A transit node lacks the destination's tree coordinates.
   FSP responds by sending a standalone CoordsWarmup (0x14) message
@@ -97,7 +97,7 @@ end-to-end encrypted) because transit nodes have no session with the source.
 ### Local Delivery
 
 When a SessionDatagram arrives with a destination node_addr matching the local
-node, FLP delivers it to FSP for session-layer processing.
+node, FMP delivers it to FSP for session-layer processing.
 
 ## Session Lifecycle
 
@@ -189,7 +189,7 @@ transport addresses or routing paths. A session survives:
 
 ### Noise IK Pattern
 
-FSP uses the same Noise IK pattern as FLP link encryption, but with
+FSP uses the same Noise IK pattern as FMP link encryption, but with
 independent keys and sessions. The full Noise descriptor is
 `Noise_IK_secp256k1_ChaChaPoly_SHA256`.
 
@@ -386,7 +386,7 @@ Cache population mechanisms:
 
 The coordinate cache maps `NodeAddr → TreeCoordinate` and is the critical
 data structure that enables efficient multi-hop routing. Without cached
-coordinates for a destination, FLP cannot make forwarding decisions and must
+coordinates for a destination, FMP cannot make forwarding decisions and must
 either fall back to bloom-filter-only routing or signal CoordsRequired.
 
 ### Unified Cache
@@ -506,13 +506,13 @@ MMP session metrics session=npub1tdwa...84le rtt=4.3ms loss=0.6% jitter=0.2ms go
 | Simultaneous initiation tie-breaker | **Implemented** |
 | Flush coord cache on parent change | **Implemented** |
 | Rekey | Planned |
-| Path MTU tracking (FLP SessionDatagram field) | **Implemented** |
+| Path MTU tracking (FMP SessionDatagram field) | **Implemented** |
 | Path MTU notification (end-to-end echo) | **Implemented** |
 
 ## References
 
 - [fips-intro.md](fips-intro.md) — Protocol overview and architecture
-- [fips-link-layer.md](fips-link-layer.md) — FLP specification (below FSP)
+- [fips-mesh-layer.md](fips-mesh-layer.md) — FMP specification (below FSP)
 - [fips-ipv6-adapter.md](fips-ipv6-adapter.md) — IPv6 adaptation layer (above FSP)
 - [fips-mesh-operation.md](fips-mesh-operation.md) — Routing, discovery, and
   error recovery