…u-gated smoothing — Principle XI. (#3975)

## Summary

Fixes #3967. Also fixes #3932. Root-caused with per-commit builds
(pre-#3836, post-#3836, 26.7.1-main, this fix) captured minutes apart
against the same live AM broadcast band on a FLEX-8400M:

![4-way
comparison](https://raw.githubusercontent.com/jensenpat/AetherSDR/pr-assets/pan-trace-regression-4way.png)

## The regression chain (two releases, two causes)

**26.6.5 — #3836's unconditional spatial smoothing ("out of focus",
#3932).** `buildFftDisplayTrace`'s 5-tap binomial smooth at a fixed 0.75
blend exists to melt the radio's RBW stair-steps when zoomed in. Applied
to every frame at every span, it rounds narrow carriers (visible height
loss on AM carriers, panel 2) and defocuses the noise floor. **Fix:**
gate the blend on the measured plateau fraction (adjacent equal-pixel
bins). Zoomed-in plateau runs (frac ≳0.65) keep the full blend — #3836's
stair-step fix is preserved; busy wide spans (frac ≲0.35) get zero — the
pre-#3836 crispness returns; the ramp between prevents a visible mode
flip while zooming.

**26.7.1 — #3958's per-pixel stroke ("spiky above and below the line",
the rest of #3967).** The stroke distance was approximated as vertical
distance scaled by a `dFdx`-derived slope — i.e. distance to the
segment's **infinite line**. On steep segments that line passes near the
entire pixel column, so the stroke drew full-height needles above and
below the trace (panel 3), and the fixed ±0.75 px smoothstep band read
as a soft fat stroke at 1× DPI. **Fix:** true endpoint-clamped distance
to the three adjacent polyline segments (4 column samples — matches the
coverage the old geometry quads had), plus falloff parity with the old
`spectrum.frag`: solid to `halfWidth−1`, 1 px fade, 1 px feather annulus
at α 0.22.

Also folds in #3968's R16F-unconditional column format as hardening
(float32 linear filtering is optional on GLES and
`isTextureFormatSupported()` can't detect filterability). Note it could
**not** have caused #3967: the reporter's own About screenshot shows
`D3D11; Intel(R) HD Graphics 520` — feature level 12.1 hardware where
R32F linear filtering is mandatory. With this folded in, #3968 can be
closed.

Not implicated (left untouched, documented for the record): #3836's
`decodeFFT` over-range renormalization only fires on radio/client
`y_pixels` mismatch (transient), and the DPR-scaled
`x_pixels`/`y_pixels` requests are orthogonal to trace sharpness.

## Why our platforms missed it

The #3958 parity checks ran on 2× Retina, where the extra AA softness is
half the logical size and the spike needles read as band activity in
static grabs. The 4-way same-band comparison above is exactly the test
that should have been run: panel 4 (this fix) restores panel 1
(pre-#3836).

## Verification

- 4-way live comparison above (same band, same radio pan, same display
settings; three grabs per build).
- `get panstats 0`: frame prep unchanged at ~121 µs/frame, 58
presents/s, `fftBuildMsPerSec` 0.28 (plateau gate skips the smoothing
copy on wide spans).
- 3D stacked-trace mode, lean mode, line-width 0, heat/solid fill:
unaffected paths re-checked by inspection; the software (non-QRhi-build)
QPainter renderer shares `buildFftDisplayTrace` and picks up the plateau
gate automatically.

## Also on the #3967 thread (separate follow-ups, not in this PR)

- "3D option no longer exists": the 2D/3D combo is the **last row of the
Display panel**, below Background — on short screens (768p laptops) it
clips off the bottom of the panadapter. Needs a scrollable/compacted
Display panel; the option itself ships fine in 26.7.1.
- The 60 fps FFT slider ceiling from #3958 is unrelated to this report
but worth an explicit release-notes mention.

💻 Generated with Claude Code (claude-fable-5 7/2/26) with architecture
by @jensenpat

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>