Implement BLE MAC Address Randomization

[BigNathan1]

Summary

Implements MAC address randomization for the simulated BLE Central to close the
passive-tracking privacy gap: a static device address let an observer correlate a
device across Bluetooth scans over time. This defines the rotation interface and
policy so both the simulation and a future Android JNI binding implement the same
contract.

Changes

• MacAddressProvider trait + RandomMacProvider (ble_transport.rs):
  injectable MAC generation. RandomMacProvider::generate() produces a 6-byte
  address with the locally-administered bit set and the multicast bit cleared.
• BleCentral rotation state (ble_transport.rs): adds current_mac,
  mac_provider, last_rotation, and rotation_interval (default 15 minutes),
  plus rotate_mac_if_due(), which rotates only once per elapsed interval and
  returns Some(new_mac) on rotation, None otherwise.
• TopologyFlag::MacRotated (types.rs): variant is now defined and emitted.
• TransportManager (unified.rs): power_tick_at() calls
  rotate_mac_if_due() and, on rotation, appends TopologyFlag::MacRotated to
  PowerTickOutcome::topology_flags so the gossip loop can broadcast the new MAC.
• notify_mac_rotated() (unified.rs): drops all BLE connections (they
  reference the stale MAC) while leaving WiFi-Direct connections intact, and keeps
  peer_count consistent.

Testing

• RandomMacProvider::generate() sets the locally-administered bit and clears the
  multicast bit over 1000 samples.
• rotate_mac_if_due() returns None before the interval and Some(_) after;
  a second immediate call returns None (no double rotation).
• TopologyFlag::MacRotated is present in power_tick output on the rotation round
  and absent before the interval elapses.
• BLE connections are dropped after notify_mac_rotated; WiFi-Direct survives.
• All transport::ble_transport tests pass.

Closes StellarConduit#38