Summary

Implements External APD support for SmartSDR 4.2.18 firmware. External APD lets the radio sample its outgoing RF from a coupled feedback path on one of the RX/XVTR inputs (rather than the internal sampler), so the predistortion engine trains against the actual transmitted signal — required when a FLEX-8x00 drives an external linear amplifier (PGXL, etc.).

The apd enable / configurable / equalizer_active plumbing was already in place. This PR adds the sampler subobject, the equalizer-reset command, and the UI.

Changes

Protocol (TransmitModel)

• New apdSampler(txAnt) getter returning {selected, available} per ANT1/ANT2/XVTA/XVTB.
• applyApdSamplerStatus(kvs) parses apd sampler tx_ant=ANT1 selected_sampler=RX_A valid_samplers=RX_A,XVTA. If selected_sampler isn't in the valid list, falls back to INTERNAL (matches FlexLib behaviour).
• setApdSamplerPort(txAnt, port) emits apd sampler tx_ant=ANT sample_port=PORT.
• resetApdEqualizer() emits apd reset.
• applyApdStatus() now also handles the bare equalizer_reset flag, emitting a new apdEqualizerResetReceived signal.
• RadioModel dispatcher routes "apd sampler" to applyApdSamplerStatus; the bare-flag form gets merged into kvs and routed to applyApdStatus.

UI (RadioSetupDialog)

• New "APD" tab between XVTR and USB Cables. Eagerly added but visibility is gated on transmitModel.apdConfigurable() and updates live via apdStateChanged, so the tab stays hidden on 6000-series radios and pre-4.2.18 firmware.
• Layout matches the SmartSDR 4.2.18 "External Sampler (per TX ANT)" group: ANT1/XVTA on top row, ANT2/XVTB on bottom, each with a port combo; Equalizer Reset button on its own row.
• Combo options + selection are kept in sync with the model via apdSamplerChanged (signal-blocked refresh, so user-driven changes don't echo back to the radio).

Tests — tests/transmit_model_apd_test.cpp, 22 assertions:

• Sampler status populates available (with INTERNAL first) and selected.
• Invalid selected_sampler falls back to INTERNAL.
• Sampler-port command formatted correctly + case-normalised.
• Empty txAnt or port silently dropped (no spurious command).
• resetApdEqualizer() emits apd reset.
• Bare equalizer_reset clears equalizer_active and fires the reset signal.
• configurable=1/=0 toggles apdConfigurable().
• Identical sampler status is idempotent (no re-emit).
• resetState() clears the sampler hash on disconnect.

Reference: Flex.Smoothlake.FlexLib v4.2.18, FlexLib/APD.cs.

Test plan

• ./build/transmit_model_apd_test — all 22 pass locally.
• Connect to a FLEX-8600 on firmware 4.2.18: APD tab visible.
• Connect to a FLEX-6000 series: APD tab not present.
• Per-antenna combo shows the radio's valid_samplers plus INTERNAL.
• Selecting RX_A for ANT1 sends apd sampler tx_ant=ANT1 sample_port=RX_A; the echoed status updates the combo without firing a second command.
• Reset button sends apd reset; subsequent status arrival clears the active indicator.

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