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- Remove outdated sections: indirect partition note, integration test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/README.md

@@ -31,15 +31,13 @@ specific topics.
 
 | Document | Description |
 | -------- | ----------- |
-| [fips-spanning-tree.md](fips-spanning-tree.md) | Spanning tree algorithms: root election, parent selection, coordinates |
+| [fips-spanning-tree.md](fips-spanning-tree.md) | Spanning tree algorithms: root discovery, parent selection, coordinates |
 | [fips-bloom-filters.md](fips-bloom-filters.md) | Bloom filter math: FPR analysis, size classes, split-horizon |
 
 ### Implementation
 
 | Document | Description |
 | -------- | ----------- |
-| [fips-software-architecture.md](fips-software-architecture.md) | Stable architectural decisions guiding the codebase |
-| [fips-state-machines.md](fips-state-machines.md) | Phase-based state machine pattern (Rust enum-of-structs) |
 | [fips-configuration.md](fips-configuration.md) | YAML configuration reference |
 
 ### Supplemental
@@ -54,16 +52,16 @@ specific topics.
                         fips-intro.md
                              │
               ┌──────────────┼──────────────┐
-              ▼              ▼              ▼
-   fips-transport-layer  fips-mesh-     fips-software-
-              │          operation       architecture
+              ▼              ▼              │
+   fips-transport-layer  fips-mesh-        │
+              │          operation          │
               ▼              │              │
-   fips-mesh-layer ◄────────┤              ▼
-              │              │       fips-state-machines
-              ▼              │
-   fips-session-layer        ├──► fips-spanning-tree
-              │              │         │
-              ▼              └──► fips-bloom-filters
+   fips-mesh-layer ◄────────┤              │
+              │              │              │
+              ▼              ├──► fips-spanning-tree
+   fips-session-layer        │         │
+              │              └──► fips-bloom-filters
+              ▼
    fips-ipv6-adapter
                         fips-wire-formats
                         (referenced by all layer docs)