MeshCore firmware frame for cmd 0x03 is
`[cmd][txt_type][channel][timestamp_le32][text]`, not `[cmd][channel][text]`.
Missing txt_type + timestamp caused every channel broadcast to come
back with ERR_UNSUPPORTED, which broke the DM-via-channel path
entirely (nothing was reaching the radio). Bring the frame into
spec — verified against meshcore-dev/MeshCore docs/companion_protocol.md.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
5616bb74e6