Rename design docs for clarity, reorganize node architecture diagram

- fips-architecture.md → fips-software-architecture.md with all
  cross-references updated (4 files)
- fips-transport-abstraction.svg → fips-node-architecture.svg, moved
  from Transport Abstraction section to Architecture Overview in
  fips-intro.md
- Added descriptive paragraph for node architecture diagram covering
  three-layer design (application interfaces, router core, transports)

docs/design/README.md

@@ -36,11 +36,11 @@ concepts, and finally the wire-level protocol details.
 
 ## Implementation
 
-| Document                                         | Description                                                      |
-|--------------------------------------------------|------------------------------------------------------------------|
-| [fips-architecture.md](fips-architecture.md)     | Software architecture: entities, state machines, configuration   |
-| [fips-tun-driver.md](fips-tun-driver.md)         | TUN interface driver: reader/writer threads, ICMPv6, packet flow |
-| [fips-state-machines.md](fips-state-machines.md) | Phase-based state machine pattern: peer lifecycle, transitions   |
+| Document                                                         | Description                                                      |
+|------------------------------------------------------------------|------------------------------------------------------------------|
+| [fips-software-architecture.md](fips-software-architecture.md)   | Software architecture: entities, state machines, configuration   |
+| [fips-tun-driver.md](fips-tun-driver.md)                         | TUN interface driver: reader/writer threads, ICMPv6, packet flow |
+| [fips-state-machines.md](fips-state-machines.md)                 | Phase-based state machine pattern: peer lifecycle, transitions   |
 
 ## Document Cross-References
 
@@ -49,7 +49,7 @@ concepts, and finally the wire-level protocol details.
                            │
               ┌────────────┴────────────┐
               ▼                         ▼
-    fips-session-protocol.md    fips-architecture.md
+    fips-session-protocol.md    fips-software-architecture.md
               │                         │
     ┌─────────┴─────────┐               ▼
     ▼                   ▼         fips-transports.md

docs/design/fips-intro.md

@@ -79,6 +79,18 @@ encryption transparently in either case.
 
 See [fips-transports.md](fips-transports.md) for transport options and characteristics.
 
+![Node Architecture](fips-node-architecture.svg)
+
+Internally, each node is organized in three layers. At the top, two application
+interfaces provide access to the mesh: a native datagram API addressed by npub,
+and an IPv6 TUN adapter that maps npubs to `fd::/8` addresses so unmodified
+IP applications can use the network transparently. The router core in the middle
+implements spanning tree maintenance, bloom filter exchange, greedy routing,
+session management, and Noise IK encryption. At the bottom, transport plugins
+handle the physical diversity — each plugin implements the same interface, so the
+router treats UDP, Ethernet, LoRa, Tor, and serial links identically. Adding a
+new transport requires no changes to the routing or session layers.
+
 ## Prior Work
 
 FIPS builds on proven designs rather than inventing new cryptography or routing
@@ -402,8 +414,6 @@ A **transport** is a physical or logical interface: a UDP socket, an Ethernet
 NIC, a Tor client, a radio modem. A **link** is a connection instance to a
 specific peer over a transport.
 
-![Transport Abstraction](fips-transport-abstraction.svg)
-
 ### Multi-Transport Bridging
 
 A node with multiple transports automatically bridges between networks. Peers
@@ -566,7 +576,7 @@ maintaining control over their own identities and peering relationships.
 | [fips-routing.md](fips-routing.md) | Bloom filters, discovery, greedy routing |
 | [spanning-tree-dynamics.md](spanning-tree-dynamics.md) | Tree protocol dynamics and convergence |
 | [fips-transports.md](fips-transports.md) | Transport protocol characteristics |
-| [fips-architecture.md](fips-architecture.md) | Software architecture, configuration |
+| [fips-software-architecture.md](fips-software-architecture.md) | Software architecture, configuration |
 
 ### External References
 

docs/design/fips-transports.md

@@ -11,7 +11,7 @@ that FIPS can operate over.
 - **Link**: A connection instance to a specific peer over a transport
 
 This document describes transport-level characteristics. See
-[fips-architecture.md](fips-architecture.md) for the Transport trait definition.
+[fips-software-architecture.md](fips-software-architecture.md) for the Transport trait definition.
 
 ## Design Principles
 
@@ -136,7 +136,7 @@ transport-layer connection before FIPS authentication can proceed.
 **Link lifecycle**: Connectionless transports use a trivial link model (no state
 machine). Connection-oriented transports require a real state machine:
 `Connecting → Connected → Disconnected`. See
-[fips-architecture.md](fips-architecture.md) for link lifecycle details.
+[fips-software-architecture.md](fips-software-architecture.md) for link lifecycle details.
 
 **Startup latency**: Connection-oriented transports add latency before a peer
 becomes usable. Tor is particularly slow (circuit setup). This affects peer
@@ -168,7 +168,7 @@ NAT devices and firewalls, limiting deployment to networks without NAT.
 ## Transport Driver Interface
 
 > **Note**: The definitive Transport trait is defined in
-> [fips-architecture.md](fips-architecture.md). This section provides a
+> [fips-software-architecture.md](fips-software-architecture.md). This section provides a
 > simplified conceptual view.
 
 Each transport driver provides:

docs/design/fips-wire-protocol.md

@@ -844,7 +844,7 @@ protocol layers apply additional policy.
 
 - [fips-intro.md](fips-intro.md) - Overall protocol design
 - [fips-session-protocol.md](fips-session-protocol.md) - Session establishment flow
-- [fips-architecture.md](fips-architecture.md) - Software architecture
+- [fips-software-architecture.md](fips-software-architecture.md) - Software architecture
 
 ### External References