Preflight

• I have read GOVERNANCE.md and confirmed this change requires an RFC (new UX + visual design + new feature area)
• I have searched existing issues and this RFC has not been proposed before
• I have not opened a PR for this change yet

Problem

ESSB / round-table SSB operators run a range of TX filter widths (e.g. 2.7k, 3.5k, 4k, asymmetric like 50–3500). To hear each station well you have to re-adjust the RX passband by hand every time the conversation moves to a different station. AetherSDR already receives the full-bandwidth panadapter FFT — it has the data to fit the filter automatically.

Proposal

An optional, per-SSB-slice "Adaptive RX filter" that measures the occupied bandwidth of the tuned signal from the panadapter FFT and continuously fits both passband edges to it — within operator-set bounds, stably (no twitching) and smoothly (glides, no abrupt jumps), and cutting off adjacent splatter so clarity of the wanted signal has priority.

• Config lives on the VFO flag's Mode tab: a checkbox + two bounds — Min low-cut {0/50/100/200, default 0} and Max high-cut {3000/3500/4000/6000, default 4000}. Adaptive never goes outside these.
• Status: the flag's filter readout shows AUTO while a live fit is applied; floor-level edge markers (SmartMTR-extreme style) sit on the panadapter filter edges.
• Fallback: when the signal is weak / not sharp / ambiguous, it applies the operator's currently selected filter preset (normal manual behavior).
• Entirely client-side: drives the existing filt command. No firmware change, no new threads, no new audio DSP. The filter stays radio-authoritative (never persisted); only the client-side enable + bounds config is persisted.

Cross-platform impact

Linux / macOS / Windows: identical. Pure Qt6/C++ client logic plus a QPainter overlay; no platform-specific code paths.

Alternatives considered

• One-shot "snap to fit" button — simpler, but needs a press per station; doesn't solve a live rotating QSO.
• More fixed presets — the status quo, and the exact pain point this addresses.

Implementation scope

• New src/core/AdaptiveFilterEngine (GUI-thread measurement/control coordinator).
• Reuses existing VoiceSignalDetector (width measurement) and SpectrumWidget noise floor.
• Small additions to SliceModel (client-side state), VfoWidget (Mode-tab controls + AUTO label), SpectrumWidget (edge markers); wiring in MainWindow_Wiring.cpp.
• Touches CMakeLists.txt for the new files (→ maintainer review).
• Known risks / needs maintainer sign-off: DSP tuning needs on-air iteration; the UX, marker visuals, label, and default bounds are design decisions for the maintainer.