What would you like?

When you grab a slice's passband and drag it to the edge of the panadapter to
tune across the band, the view should keep scrolling smoothly so you can sweep
the whole range in one gesture. Today you effectively have to zoom in first.
Reported by a user as a feature wish.

Related: this is the mouse drag-to-tune method floated in #1474 (umbrella for
"additional ways to change slice frequency"), but as a refinement to the
existing drag rather than a new method. Touches the same mouseMoveEvent
surface as #3444 (drag event overload) — kept separate from that bug.

Current behaviour (measured)

A slice drag already pans a little at the edge, but it creeps and stutters
("rubber band", harder going down than up):

• The slice drag emits incrementalTuneRequested per mouse-move; the edge
  follow is revealFrequencyIfNeeded, a position controller — it nudges the
  slice a bit past the trigger margin (kIncrementalTriggerEdgeMarginFrac=0.05).
• At the border the cursor can't move past the frame, so the overshoot that
  drives the nudge is tiny and self-limiting (as the center follows, the
  cursor frequency follows with it). Measured: ~0.0027 MHz/tick regardless
  of how hard you push → ~0.1×span/s. Crossing a band takes ~20 s.
• The flag-extended trigger (VFO flag outer-edge should trigger Pan Follow VFO (so flags don't clip in split mode near pan edge) #2761) fires asymmetrically just inside the edge on
  the flag side, so reveal kicks in before a velocity takeover and fights it →
  the one-sided stutter/jump-back.

Proposed behaviour (implemented, validated)

A velocity controller for the edge: while the cursor sits in the edge zone,
a ~30 Hz timer pans at a speed that scales with edge depth and ramps up with
hold time, keeping the slice parked just inside the leading edge so it stays
visible while the band scrolls under it. The pan+tune go through a dedicated
path that bypasses reveal (so it can't fight). Measured after the fix:
dMhz=0.00182 /tick at full depth+ramp = 1.2×span/s, center monotonic, no
jump-back, symmetric L/R; slice parked at the 5% boundary (sliceX=243/4617).

AETHER_NO_DRAG_EDGEPAN=1 restores the legacy reveal-only behaviour for A/B.
Velocity defaults (top speed / ramp / interval) are exposed as env overrides so
the feel can be tuned without a rebuild — these defaults are a UX decision
and need maintainer sign-off (AGENTS.md: maintainer is the sole UX authority).

Notes

This is a UX behaviour change, so per AGENTS.md it is raised as an issue for
maintainer direction first; the PR notes the velocity defaults need review.