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.

_common.py

@@ -0,0 +1,36 @@
+"""Shared defaults for the example scripts.
+
+MeshCore's "USA/Canada (Recommended)" preset, per
+https://github.com/ripplebiz/MeshCore/blob/main/docs/faq.md §6.1
+(the "narrow" channel the project moved to in October 2025):
+
+* 910.525 MHz — inside the 902-928 MHz ISM band
+* 62.5 kHz bandwidth
+* SF 7 — lower SF for faster transmissions and better SNR headroom
+* CR 4/5 — RadioLib's ``LORA_CR=5``
+* 16-symbol preamble — matches every board variant's ``radio.begin(...)``
+* Sync word 0x1424 — ``RADIOLIB_SX126X_SYNC_WORD_PRIVATE``, the SX126x
+  register encoding of the "private" byte 0x12 MeshCore uses to
+  distinguish its traffic from LoRaWAN (byte 0x34, encoded 0x3444).
+* 20 dBm TX power — MeshCore's compile-time default (see
+  ``examples/companion_radio/MyMesh.h``).
+
+If you operate in the EU, swap ``freq_hz`` for ``869_525_000`` (the
+EU-equivalent MeshCore channel) and cap ``tx_power_dbm`` at ``14`` to
+respect the 868 MHz duty-cycle / ERP rules.
+"""
+
+import donglora as dl
+
+MESHCORE_US = dl.LoRaConfig(
+    freq_hz=910_525_000,
+    sf=7,
+    bw=dl.LoRaBandwidth.KHZ_62_5,
+    cr=dl.LoRaCodingRate.CR_4_5,
+    preamble_len=16,
+    sync_word=0x1424,
+    tx_power_dbm=20,
+    header_mode=dl.LoRaHeaderMode.EXPLICIT,
+    payload_crc=True,
+    iq_invert=False,
+)