Design documentation illustration and review pass

Wire format diagrams:
- Add 24 SVG diagrams covering every FMP and FSP wire format: common
  prefix, established frame headers, Noise IK handshake messages,
  handshake flow, TreeAnnounce, AncestryEntry, FilterAnnounce,
  LookupRequest/Response, SessionDatagram, Disconnect, SenderReport,
  ReceiverReport, FSP complete message, SessionSetup/Ack/Msg3,
  PathMtuNotification, CoordsRequired, PathBroken, and MtuExceeded
- Replace ASCII art in fips-wire-formats.md with SVG references
- Apply text edits to fips-mesh-layer.md, fips-mesh-operation.md,
  fips-transport-layer.md, and fips-ipv6-adapter.md

Spanning tree dynamics:
- Add 12 topology SVG diagrams: node join (overview + 3-panel steps),
  three-node convergence (4-panel), link addition with depth labels,
  link removal, partition formation, and 6 real-world example diagrams
  (office, mixed-link, two-site WAN topologies)
- Rewrite all code blocks to narrative prose with diagram references
- Add inline prior art attributions distinguishing Yggdrasil-derived
  concepts from FIPS-novel contributions
- Add 3 new references (De Couto ETX, IEEE 802.1D, RFC 2328 OSPF) and
  Prior Art summary
- Remove outdated sections: indirect partition note, integration test
  gaps, DHT-based lookup reference
- Change "must elect a new root" to "must rediscover its new root"

Spanning tree design review (fips-spanning-tree.md):
- Rename "Root Election" to "Root Discovery" across docs
- Add "What Is a Spanning Tree?" introductory section
- Add parent selection intro explaining self-organization role
- Fix tree distance example: 4 hops, not 2
- Clarify timestamp field as advisory only
- Remove unimplemented ROOT_TIMEOUT and TREE_ENTRY_TTL from timing
  parameters and implementation status tables

Bloom filter design review (fips-bloom-filters.md):
- Add "What Is a Bloom Filter?" intro section
- Rewrite Purpose section to frame filters as routing path identification
- Correct FPR analysis (old values were 3-50x overstated)
- Add Filter Occupancy Model based on network size and tree position
- Fix filter expiration to describe actual MMP-based cleanup
- Combine Scale Considerations with Size Classes after Wire Format
- Fix stale FPR values in src/bloom/mod.rs comments

Session layer review (fips-session-layer.md):
- Add inline prior art attributions: Noise Protocol Framework, WireGuard,
  DTLS (RFC 6347), IKEv2 (RFC 7296), RFC 1191 PMTUD, Yggdrasil, NIP-44
- Replace warmup state machine ASCII art with SVG diagram
- Convert CoordsWarmup wire format code block to prose
- Add External References section with full citations

Level 5 implementation doc cleanup:
- Delete fips-software-architecture.md (redundant with protocol layer docs)
- Delete fips-state-machines.md (Rust tutorial, not protocol design)
- Add fipsctl command reference to README.md
- Update cross-references in fips-intro.md, docs/design/README.md,
  fips-transport-layer.md, fips-configuration.md

Fixes:
- Correct fd::/8 to fd00::/8 in fips-session-layer.md,
  fips-identity-derivation.svg, and fips-node-architecture.svg
- Fix config example MTU: 1197 → 1472 in fips-configuration.md

File organization:
- Move all SVG diagrams into docs/design/diagrams/ subdirectory
- Update all diagram references to use new paths

docs/design/fips-transport-layer.md

@@ -16,7 +16,7 @@ to the FIPS Mesh Protocol (FMP) above.
 The transport layer deals exclusively in **transport addresses** — IP:port
 tuples, MAC addresses, .onion identifiers, radio device addresses. These are
 opaque to every layer above FMP. The mapping from transport address to FIPS
-identity happens at the link layer after the Noise IK handshake completes.
+identity happens at the link layer after the Noise IK link handshake completes.
 The word "peer" belongs to the link layer and above; the transport layer
 knows only about remote endpoints identified by transport addresses.
 
@@ -69,7 +69,7 @@ forwarding and LookupResponse transit annotation.
 
 For connection-oriented transports, manage the underlying connection: TCP
 handshake, Tor circuit establishment, Bluetooth pairing. FMP cannot begin
-the Noise IK handshake until the transport-layer connection is established.
+the Noise IK link handshake until the transport-layer connection is established.
 
 Connectionless transports (UDP, raw Ethernet) skip this — datagrams can flow
 immediately to any reachable address.
@@ -133,7 +133,7 @@ and duplication at the routing layer.
 
 **Connection model**: Connectionless transports (UDP, raw Ethernet) allow
 immediate datagram exchange. Connection-oriented transports (TCP, Tor, BLE)
-require connection setup before FMP can begin the Noise IK handshake,
+require connection setup before FMP can begin the Noise IK link handshake,
 adding startup latency.
 
 **Stream vs. datagram**: Datagram transports have natural packet boundaries.
@@ -422,5 +422,3 @@ quality difference is significant. Link cost is not yet used in
 - [fips-intro.md](fips-intro.md) — Protocol overview and layer architecture
 - [fips-mesh-layer.md](fips-mesh-layer.md) — FMP specification (the layer above)
 - [fips-wire-formats.md](fips-wire-formats.md) — Transport framing details
-- [fips-software-architecture.md](fips-software-architecture.md) — Transport
-  trait implementation details