Support KISS ACKMODE end to end; pin samoyed to the ACKMODE build

[M0LTE]

net-sim itself never parses KISS frames — the host talks KISS straight to each samoyed child, and the router only moves PCM audio — so "ackmode support" here is twofold: build a samoyed that implements ACKMODE, and prove the round trip works through a real child.

What ACKMODE is

The G8BPQ extended-KISS ACKMODE command (nibble 0x0C, per the Multi-Drop KISS spec): the host sends a data frame with two opaque id bytes between the command byte and the AX.25 frame (C0 xC aa bb <frame> C0); once the frame has actually been transmitted, the TNC echoes those two bytes back (C0 xC aa bb C0). This lets a host start its FRACK/retry timers from the real on-air moment, which matters on HF. Previously samoyed refused the opcode outright.

Changes

• Pin samoyed to the ACKMODE build. New SAMOYED_REPO + SAMOYED_REF build args in the Dockerfile, defaulted (with matching install.sh defaults) to M0LTE/samoyed @ 6b4f5c7 — the ACKMODE work, which currently lives on a samoyed fork branch and is not yet upstreamed to doismellburning/samoyed. Pinned to a fixed SHA so release images are reproducible, with a clear revert-to-upstream path in the comments for when ACKMODE lands upstream. The docker workflow no longer resolves a floating samoyed main HEAD — the Dockerfile is now the single source of truth for which samoyed gets built.
• internal/tnc/ackmode_test.go — an end-to-end integration test: it spawns a real samoyed child, dials its KISS TCP port, sends an ACKMODE data frame, and asserts samoyed transmits it and echoes the same two id bytes back with command nibble 0x0C. It skips (doesn't fail) when no samoyed binary is present, so the suite stays green where samoyed isn't built; point it at one with SAMOYED_BIN (the release build, or make -C samoyed cmds).
• README — ACKMODE moves from "Known limitations" to supported; the install env-var table documents the SAMOYED_REPO/SAMOYED_REF pin.

Verification

• make test (vet + go test ./...) green; the integration test skips without a binary.
• With the pinned samoyed binary, the integration test passes: samoyed accepts the frame, logs [0L] Q1TEST>Q2TEST:ackmode round-trip, and the ack returns to the originating client with nibble 0x0C and the right id bytes.

Once this merges, tagging v0.1.0 produces the first published ghcr.io/packethacking/net-sim release image with ACKMODE available.