feat(mesh) P1: trios-mesh M2 — TUN/netdev IP-over-radio with real ETX metric

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Goal

Run ordinary IP apps (ping, iperf3, later MAVLink C2) over the 5.8 GHz mesh: open a Linux TUN on each Mini, route packets by a real ETX metric, encrypt over the radio socket, decrypt back into TUN.

Context

Honest status: trios-mesh does not exist as a repo or on disk. Its crypto/routing pass unit tests in simulation only — never on hardware [-sim]. This issue is trios-mesh MILESTONE 2 and is greenfield.

Board: P201/P203 Mini (Zynq-7020 xc7z020, dual Cortex-A9, AD9361 SDR) — the flying node. The daemon runs on the Mini's PS-side ARM-Linux (aarch32 Cortex-A9), not on the AX7203 (no hard CPU, too heavy to fly). Cross-compile target: arm-linux — this exercises real sockets + TUN ioctls the sim never touched.

Now that the FPGA is physically connected and P0 gates the first flash, M2 sits directly on top of the PHY: #9 must expose a radio socket / sample interface for the daemon to send/recv frames. Prior milestone #10 (M1: repo + X25519 handshake + ChaCha20-Poly1305 session on real ARM-Linux) provides the crypto session this milestone encrypts with.

Tasks

• daemon/: open a Linux TUN device via /dev/net/tun TUNSETIFF ioctl; assign mesh IP from 10.42.0.0/24 (one per node); bring if up.
• Read IP packets off TUN; parse dst; look up next-hop in the routing table.
• routing/: implement a real ETX metric — periodic HELLO probes, per-neighbor delivery-ratio (df·dr) measurement, ETX table, best-next-hop selection. Replace any hop-count stub.
• Wire send path: on TX, AEAD-encrypt the IP frame with the ChaCha20-Poly1305 session from M1 (crypto/, 96-bit nonce = counter+direction, never reused; replay window), then write to the feat(fpga) P1: AD9361 5.8GHz TX/RX + OFDM PHY (single-carrier fallback) #9 radio socket. MTU-aware framing (account for AEAD tag + mesh header).
• Wire recv path: read frame from radio socket → decrypt/verify tag → inject back into TUN.
• smoke/: 2-node real-hardware smoke — bring up TUN on both Minis, ping across the encrypted link, then iperf3 over 1 hop. Not a unit test.
• 3-node relay: iperf3 across a middle node = 2 hops (this is M3's gate, but wire the relay path here).
• Cross-compile the daemon to arm-linux Cortex-A9; document the build in the repo README.
• Log the run (TUN ifname, mesh IPs, ETX table snapshot, throughput) into fpga/FLASH_HISTORY.md-style record in the trios-mesh repo.

Acceptance criteria

• ip addr on each Mini shows a tun0-class device with a 10.42.0.x/24 mesh IP, link UP.
• ping 10.42.0.<peer> succeeds across the radio link; tcpdump on the radio socket shows only ChaCha20-Poly1305 ciphertext (no plaintext IP).
• iperf3 reports a non-zero throughput baseline over 1 hop, and > 0 Mbit over 2 hops through a relay node.
• ETX table is populated from live HELLO probes (delivery ratio, not hop-count); killing a link changes the selected next-hop (full self-heal is M5, but next-hop must react here).
• Any AEAD tag mismatch / replayed nonce is dropped and logged (no plaintext injected into TUN).

Dependencies

• blocked_by: feat(mesh) P1: scaffold trios-mesh repo + M1 X25519/ChaCha20 on real ARM (Mini) #10 — trios-mesh MILESTONE 1: repo + X25519 handshake + ChaCha20-Poly1305 session on real ARM-Linux.
• blocked_by: feat(fpga) P1: AD9361 5.8GHz TX/RX + OFDM PHY (single-carrier fallback) #9 — AD9361 5.8 GHz OFDM PHY exposing the radio socket / sample interface this daemon sends/recvs over.

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phi^2 + phi^-2 = 3

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M2 data plane implemented in trios-mesh PR (ETX next-hop + hop-by-hop ChaCha20-Poly1305 forwarding + 2-hop relay). 26 unit + 2 integration green, clippy clean. Host -sim; real /dev/net/tun + radio transport graduate on the Mini (#8).