…absolute tune (#3260). Principle XI.

FlexControl knob produced a residual +60 Hz offset on every tune step on
macOS (and silently on Windows/Linux). Root cause is the persistent
encoder target m_flexTargetMhz interacting with the #1524 1 kHz
jitter-suppression tolerance plus the lack of step-grid alignment when
re-initializing the base.

Three coordinated edits in MainWindow.cpp, all mirroring patterns that
already work on the MIDI tune-knob path:

1. applyFlexControlWheelAction: when (re)initializing m_flexTargetMhz
   from s->frequency(), snap to the nearest step-Hz multiple instead of
   copying verbatim. This is the same `(curHz + stepHz/2) / stepHz *
   stepHz` snap the MIDI rx.tuneKnob path uses, so encoder ticks land
   on clean grid values even when the slice is off-grid.

2. RxApplet::stepSizeChanged hook: invalidate m_flexTargetMhz,
   m_midiTuneTargetMhz and stop their coalesce/idle timers when the
   user changes step size. This forces the next encoder tick to rebase
   + re-snap against the new step grid (va3mw's repro: step 20 Hz →
   500 Hz then turn knob).

3. applyTuneRequest: on CommandedTargetCenter and AbsoluteJump intents
   (typed VFO entry, spectrum click, spot/bandstack recall), reset
   encoder accumulators and stop their timers. The #1524 1 kHz
   tolerance otherwise keeps a stale m_flexTargetMhz alive whenever the
   typed frequency lands inside that window, producing the OP repro
   (type 7.188.000 with stale base 7.187.960 + 100 Hz step → 7.188.060).

The #1524 jitter-suppression behavior during sustained knob spinning is
preserved: incremental encoder ticks still skip the reset path and the
1 kHz tolerance still gates mid-tune rebases from lagging radio echoes.

Blast radius: applyFlexControlWheelAction risk_score=0.4587,
11 high-risk affected (top: MainWindow::MainWindow,
AppSettings::instance, AppSettings::value). All downstream calls are
existing member-variable reads/writes (m_flexTargetMhz,
m_flexCoalesceTimer, m_midiTuneTargetMhz, m_midiTuneIdleTimer); no
signatures, link surface, or call graph topology changed. Upstream
callers — handleFlexControlTuneSteps and handleVirtualFlexControlWheel —
are unchanged.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>