The packet CSV dump only exposed hop limit, which is enough to map whole-network coverage but not to tell which repeater relayed a given packet. This surfaces the relaying repeater (and the original hop count) so users can map the coverage of individual repeaters.

Resolves #5801

🌟 New Features

• Add a relay node column carrying the relay_node byte from MeshPacket — the last byte of the relaying node's NodeNum. It's emitted as a 2-digit hex value so it matches the tail of a node id (e.g. !a1b2c3d4 → "d4"), and left blank when unset (0). This is the "last hop byte" requested in the issue: it identifies the repeater a packet was heard from.
• Add a hop start column alongside the existing hop limit so a reader can derive hops-away (hop_start - hop_limit) and confirm whether a packet was a direct vs. relayed reception.

🛠️ Notes on column ordering

• Both new columns are inserted before payload, keeping the free-text payload field last (safest for CSV with embedded commas/quotes) and leaving every pre-existing column at its current index. Tools that read columns by header name are unaffected; tools that read payload by fixed position will need to shift it two columns right.

Testing Performed

• Updated ExportDataUseCaseTest (core/domain commonTest):
  • invoke writes header to sink now asserts the new "hop start","relay node","payload" header tail.
  • invoke writes hop start and relay node columns — verifies a packet with hop_limit=2, hop_start=3, relay_node=77 serializes as "2","3","4d","Hello".
  • invoke leaves relay node blank when unset — verifies relay_node=0 produces a blank field.
• Full baseline: ./gradlew spotlessApply spotlessCheck detekt assembleDebug test allTests — all pass.