Add BLE L2CAP transport with scan-based auto-connect

BLE transport implementation using L2CAP Connection-Oriented Channels
(SeqPacket mode) via the bluer crate, behind cfg(feature = "ble").

Core transport:
- BleTransport<I> generic over BleIo trait (BluerIo prod, MockBleIo test)
- Connection pool with priority eviction (static > discovered, max 7)
- Connect-on-send via connect_inline() matching TCP behavior
- Per-connection receive loops with pool cleanup on disconnect

Discovery and probing:
- Combined scan_probe_loop using select! over scanner events and a
  BinaryHeap delay queue with per-entry random jitter (0-5s) to prevent
  herd effects when multiple nodes see the same beacon simultaneously
- Pre-handshake pubkey exchange ([0x00][pubkey:32]) for IK identity
- Cross-probe tie-breaker: smaller NodeAddr's outbound wins (same
  convention as FMP/FSP rekey dual-initiation)
- Probed peers reported to DiscoveryBuffer; pool fills through normal
  node-layer auto-connect -> send_async -> connect_inline path

Beacon management:
- Periodic advertising: 1s burst every 30s (configurable via
  beacon_interval_secs / beacon_duration_secs)
- FIPS service UUID for scan filtering

Configuration (all fields optional with defaults):
- adapter, psm, mtu, max_connections, connect_timeout_ms
- advertise, scan, auto_connect, accept_connections
- beacon_interval_secs (30), beacon_duration_secs (1)

Hardware validated with two BLE nodes:
- 2048-byte MTU, ~60-160ms RTT, zero-config auto-connect
- BLE spike tool at testing/ble/ for standalone adapter validation

42 unit tests + 4 node-level integration tests, all CI-compatible
via MockBleIo (no hardware required). tokio test-util added for
time-dependent scan/probe tests.

src/node/tests/mod.rs

@@ -5,6 +5,8 @@ use crate::PeerIdentity;
 use std::time::Duration;
 
 mod bloom;
+#[cfg(target_os = "linux")]
+mod ble;
 mod disconnect;
 mod discovery;
 #[cfg(target_os = "linux")]