…margin (#2941). Principle XI.

Single-click on the off-screen slice indicator (the "C> 3.777" / "<B 3.573"
chevrons in the panadapter corner) reaches MainWindow::revealFrequencyIfNeeded
with TuneIntent::RevealOffscreen, where the comfort margin was hard-coded to
0.0 — settling the slice exactly on the visible edge.  The result is partially
clipped and indistinguishable from "still off-screen" to the user, which is
the behavior #2371 / #2941 reports.

Switch the RevealOffscreen branch to kRevealComfortEdgeMarginFrac (0.18, the
same constant used for non-click reveals), so the slice lands ~18% in from the
edge instead of straddling it.  Smallest defensible fix per the issue triage
(Option B); does not alter the documented single-click vs. double-click split
introduced by #2272 (double-click still hard-centers).

The previous closure of #2371 ("Closed in #2272") was premature: #2272 added
the double-click hard-center path but left the single-click reveal-to-edge
behavior intact.  Citing Principle XI: that earlier "Fixes #2371" claim was
undemonstrated against the original reporter's repro, so the bug came back
under a new number; this PR is the actual demonstration.

Blast radius: risk_score=0.484, 14 high-risk affected (top: AppSettings::instance,
MainWindow::MainWindow, AppSettings::value).  Reviewed: the only call site
passing TuneIntent::RevealOffscreen is MainWindow::setActiveSliceInternal
(src/gui/MainWindow.cpp:10721), which is itself only triggered by the
SpectrumWidget::sliceClicked signal — i.e. the off-screen-indicator click
flow.  Other callers of revealFrequencyIfNeeded pass CommandedTargetCenter
(hard-center, returns early) or IncrementalTune (uses its own incremental
margins, ignores comfortMargin), so the wide upstream caller surface is
unaffected by this change.

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