Rewrite all scripts against the DongLoRa Protocol v2 async Dongle surface, and extract the shared MeshCore US preset into a single module.
* _common.py: MESHCORE_US preset (910.525 MHz / BW 62.5 / SF 7 / CR 4/5 / sync 0x1424 / 20 dBm) plus small helpers, imported by every example.
* New: explore.py, which prints the device capabilities and runs a single TX + RX roundtrip for quick hardware validation.
* simple_rx.py, simple_tx.py, ping_pong.py, lora_bridge.py all switch to donglora.Dongle and the new Modulation / LoRaConfig types.
* all_commands.py removed; it was an index of the old v0.1 command enum that no longer exists after the Protocol v2 split.
* justfile: drop stale recipes that referenced removed scripts.
