Implement the WireGuard data plane plugin

The first IP plugin, built on the data plane boundary the core already had.

Plugin:
- one X25519 key per network in the plugin's own wireguard.sqlite, separate
  from the iroh identity and from the network secret; a damaged store is an
  error, never a silently regenerated identity
- deterministic IPv6 ULA overlay: every member derives the same /64 from the
  network id and its own /128 from its WireGuard public key, so no
  coordinator allocates addresses
- AllowedIPs are derived locally, never taken from a peer's announcement, so
  a member cannot claim another member's overlay address; a mismatched claim
  is rejected
- bounded, versioned, validated announcement carried as the existing opaque
  capability payload, which the core still never parses
- each agent builds its own full-mesh configuration (N-1 peers) and
  reconciles on every change and on a timer, repairing drift
- WireguardBackend abstraction: RecordingBackend in memory, and WgToolBackend
  driving real wg/ip on Linux, split into a pure planner plus parsers and a
  thin executor so everything interesting is testable without root

Core, three generic additions the plugin needed:
- IpPlugin::on_network_activated, so per-network state is ready before peers
- PluginContext for re-announcements and error reports from plugin tasks,
  with errors counted by the owning network runtime
- IpPlugin::shutdown, awaited with a grace period, so system objects go away

94 tests pass offline with no privileges: 35 new WireGuard unit tests and 12
integration tests over real iroh connections. The real wg/ip backend needs
root and is behind --ignored in tests/wireguard_system.rs; it was not run.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/sync-model.md

@@ -2,7 +2,9 @@
 
 **Nothing in this document is implemented.** The proof of concept exchanges
 hostname and capability announcements over live sessions and keeps no
-replicated history. This file records the intended direction so the module
+replicated history. That is also why WireGuard peer membership is
+session-scoped today: a peer disappears from the overlay configuration when its
+control session ends, because there is no agreed durable state to keep it. This file records the intended direction so the module
 boundaries in [architecture.md](architecture.md) stay compatible with it, and so
 nobody mistakes the current announcements for synchronisation.