M3 step 2: Meshtastic header and presets, pcap with LoRaTap (host-tested)

The 16-byte clear header (hops away, channel hash, relay node), the EU_868
presets and their frequency slots, and the channel hash, all checked
against Meshtastic's source. Captures are pcap with LoRaTap v0, read back
with TShark 4.2.5; packet RSSI is plain dBm, as Wireshark reads it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

lib/meshtastic/src/meshtastic_header.h

@@ -0,0 +1,36 @@
+#pragma once
+
+#include <cstddef>
+#include <cstdint>
+#include <string>
+
+namespace roro::meshtastic {
+
+constexpr uint32_t kBroadcast = 0xFFFFFFFF;
+
+// The 16 bytes every Meshtastic packet starts with, sent in clear (little-endian). The payload
+// after it is encrypted with the Channel's key.
+struct PacketHeader {
+    uint32_t to = 0, from = 0, id = 0;
+    uint8_t flags = 0;
+    uint8_t channelHash = 0;  // the Channel's name and key folded into a byte (see channelHash())
+    uint8_t nextHop = 0;      // last byte of the Node meant to relay it next; 0 when flooding
+    uint8_t relayNode = 0;    // last byte of the Node that relayed it to us
+
+    uint8_t hopLimit() const { return flags & 0x07; }
+    bool wantAck() const { return flags & 0x08; }
+    bool viaMqtt() const { return flags & 0x10; }
+    uint8_t hopStart() const { return flags >> 5; }
+    bool broadcast() const { return to == kBroadcast; }
+    // How many times it was relayed before we heard it; -1 when the sender didn't say (hop start 0).
+    int hopsAway() const { return hopStart() == 0 ? -1 : hopStart() - hopLimit(); }
+};
+
+constexpr size_t kHeaderSize = 16;
+
+bool parseHeader(const uint8_t* data, size_t len, PacketHeader& out);
+
+// "!12345678", as Meshtastic writes node numbers; "all" for broadcast.
+std::string nodeId(uint32_t node);
+
+}  // namespace roro::meshtastic