docs: implementer's protocol guide

[M0LTE]

Summary

• New `docs/implement.md` for people writing a second-source DAPPS implementation (a node, relay, embedded build).
• Two-part structure as requested: bare essentials (smallest interop set) + full feature-by-feature (F1, F2, ordering, TTL, headers, peers, rev, datagram codec, beacons, solicits).
• Annotated on-air examples for every flow, with file:line citations into the reference daemon.
• Compliance smoke-test checklist at the end so an implementer can verify against the C# daemon.
• Cross-linked from `app-developers/reference.md` (the existing wire-format summary now points here for the full spec) and `docs/index.md` (Status section).

Test plan

• `mkdocs build --strict` clean (no warnings, all internal links resolve)
• All file:line citations verified against current `master`
• Two new wire-format details corrected against canonical code: `chk` is lowercase 4-hex (`Crc16CcittFalse.ComputeHex` uses `x4`); the `?` "already have" reply mentioned in the old summary table doesn't exist in the inbound handler - the page documents only the responses actually emitted (`send`, `ack`, `bad`, `error`, `eh?`)
• Page renders into mkdocs nav between "App developers" and "Troubleshooting"

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