Context

FlexRadio shipped FlexLib v4.2.18.41174 today. New maritime NAVTEX TX waveform with its own status object, command, and slice mode tag. AetherSDR's mode-string switch must not crash on NT.

Reference: ~/build/FlexLib/FlexLib/NAVTEX.cs and Radio.cs ParseStatus.

What the radio does now

• Broadcasts S<handle>|navtex sent idx=<N> serial_num=<N> per transmitted message.
• Broadcasts S<handle>|navtex status=<Inactive|Active|Transmitting|QueueFull|Unlicensed|Error>.
• Forces a single slice into mode NT at 518 / 490 / 4209.5 kHz when activated.
• Accepts navtex send tx_ident=<C> subject_indicator=<C> [serial_num=<N>] msg_text="..." — replies with R<seq>|0|<idx> mapping the index to the message.

Work for AetherSDR

• Add sub navtex all to the connect sequence.
• Parse navtex sent and navtex status into a new NavtexModel.
• Send navtex send ... from a new applet (compose form with tx_ident, subject_indicator, message).
• Make SliceModel mode-string switch tolerant of NT (treat as digital data mode for filter/AGC purposes).
• Display NAVTEX status (Active/Transmitting/QueueFull etc.) in the new applet.

Visual design + UX (applet placement, fields, etc.) is maintainer-only — implement the data model + protocol plumbing and stop, per CLAUDE.md.