Background

After PR #3030 (fix for #2941) changed the revealOffscreen branch to return kRevealComfortEdgeMarginFrac instead of 0.0, the ternary at src/gui/MainWindow.cpp:12238-12241 has redundant first/third branches that both return the same value:

```cpp
const double comfortMargin =
revealOffscreen ? kRevealComfortEdgeMarginFrac // <-- same...
: isSpectrumClick ? kSpectrumClickEdgeMarginFrac
: kRevealComfortEdgeMarginFrac; // <-- ...as this
```

Proposed cleanup

Collapse to:

```cpp
const double comfortMargin = isSpectrumClick
? kSpectrumClickEdgeMarginFrac
: kRevealComfortEdgeMarginFrac;
```

The revealOffscreen variable is still needed for triggerEdgeMarginFrac a few lines later, so leave the computation in place — just drop it from the comfortMargin ternary.

Pull the explanatory comment forward from #3030 with a note that off-screen-indicator clicks share the default reveal margin, since the explicit branch is going away.

Why this is a follow-up, not a fix-up to #3030

• fix(spectrum): off-screen slice indicator click reveals with comfort margin (#2941). Principle XI. #3030 was a behavior fix; this is a stylistic cleanup. Keeping them separate makes both easier to bisect if anything ever regresses.
• Trivially mergeable on its own, doesn't block anything.
• Spotted during review of fix(spectrum): off-screen slice indicator click reveals with comfort margin (#2941). Principle XI. #3030 (review thread).

Effort

~2 minutes. Single-file, single-hunk, no test impact.