BLE continuous advertising, probe cooldown replaces burst beacon

Replace burst beacon pattern (1s on / 30s off) with continuous
advertising. The burst pattern caused L2CAP connect timeouts because
the remote side was no longer connectable when the probe fired after
jitter delay. BLE advertising overhead is negligible (~0.15% duty
cycle on advertising channels).

Replace the seen HashSet + jitter delay queue with a simple cooldown
map. After probing an address (success or failure), suppress re-probe
for 30s (configurable via probe_cooldown_secs). Connected peers are
filtered by pool membership check. This eliminates the bug where
failed probes permanently blacklisted addresses for the session
lifetime.

Remove config fields: scan_interval_secs, beacon_interval_secs,
beacon_duration_secs. Add: probe_cooldown_secs.

CHANGELOG.md

@@ -21,9 +21,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Bluetooth Low Energy (BLE) L2CAP Connection-Oriented Channel transport
   with per-link MTU negotiation, behind the `ble` Cargo feature flag
   (default-on, Linux only, requires BlueZ)
-- BLE peer discovery via scan/probe with per-entry random jitter to
-  prevent herd effects when multiple nodes see the same beacon
-- Periodic BLE beacon advertising with configurable interval and duration
+- BLE peer discovery via continuous scan/probe with cooldown-based
+  deduplication (`probe_cooldown_secs`, default 30s)
+- Continuous BLE advertising for reliable L2CAP connectivity
 - Cross-probe tie-breaker using deterministic NodeAddr comparison
 - Connection pool with configurable capacity and eviction