feat!: examples on donglora 1.0 async API with shared _common helpers

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.

justfile

@@ -34,9 +34,9 @@ tx *args: _ensure_tools
 ping-pong *args: _ensure_tools
     @{{_uv}} {{source_directory()}}/ping_pong.py {{args}}
 
-# Exercise all DongLoRa commands
-test-commands *args: _ensure_tools
-    @{{_uv}} {{source_directory()}}/all_commands.py {{args}}
+# Probe the connected DongLoRa: dump GET_INFO and exercise one TX + RX
+explore *args: _ensure_tools
+    @{{_uv}} {{source_directory()}}/explore.py {{args}}
 
 # Two-way LoRa bridge over TCP
 bridge *args: _ensure_tools