#3583 landed on main after the release branch was cut; it ships in v26.6.3, so
record it in the release notes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

…d waterfall floor (#3586)

## Summary

Makes the waterfall noise floor render as a flat, evenly-levelled band
by using the **radio's own per-tile auto-black level** instead of a
client-side estimate.

AetherSDR was computing its own noise-floor estimate and sending the
radio `auto_black=0`, which produced a textured, black-speckled floor.
The radio already computes a noise-floor black level and embeds it in
every waterfall tile (FlexLib's `WaterfallTile.AutoBlackLevel`) —
AetherSDR even parsed it (`PanadapterStream.cpp`) and emitted
`waterfallAutoBlackLevel()`, but the value was discarded (the handler
clamped the raw uint16 to 0–125).

This wires it up:

1. `RadioModel` sends `auto_black=1` so the radio populates the tile
field.
2. `MainWindow_Session` feeds the per-tile value into the renderer via
the new `SpectrumWidget::setRadioAutoBlackLevel()`.
3. `intensityToRgb` maps `low` = radio auto-black, `high` =
`wfHighThresholdRaw()` — a cubic colour-gain curve:
   ```
   num  = (100 − colorGain)/100 · cbrt(65535 − low)
   high = low + num³          (floored at low + 100)
   ```
then `t = (rawValue − low) / (high − low)` → gradient. The client-side
estimate stays as a fallback (FFT fallback / auto-black off).

Both display paths are covered — the native waterfall is coloured on the
CPU via `intensityToRgb`, then uploaded as a texture (no GPU-side
mapping).

## Reviewer note

Try it with **Waterfall (Color) Gain ≈ 30** and **Auto Black Level ≈
30** — it works very well, particularly with the new **Purple** colour
scheme (#3583). At default Color Gain and Black Level offset = 50 (no
bias), the floor follows the radio's computed level directly.

## Constitution principle honored

**Principle I — FlexLib Is The Protocol Authority.** Replaces a
client-side approximation with the radio's own auto-black value
(FlexLib's per-tile `AutoBlackLevel`), the authoritative noise-floor
figure.

It also **reverses a deliberate client-side decision** and changes a
rendering default for all waterfall schemes, so it is flagged for
**maintainer sign-off** on the look (Principle XIII — the
operator/maintainer is the authority on visual design).

## Test plan

- [x] Local build passes (`cmake --build build`) — links clean
- [x] Verified on a live FLEX-8600 (fw 4.2.20.41343) — floor now evenly
levelled
- [ ] Existing tests pass (CI)
- [ ] Maintainer review of the architecture reversal + visual default

## Open questions for review

- The auto-black **offset** slider now biases the radio black point
(`(50 − offset)·0.5·128` raw); scaling is open to tuning.
- Initial waterfall config sends `auto_black=1` unconditionally; could
instead reflect the persisted client auto-black state.

## Checklist

- [x] No new flat-key `AppSettings` calls (none added)
- [x] All meter UI uses `MeterSmoother` — N/A (no meters touched)
- [x] Commits are signed (SSH)
- [x] Security-sensitive changes reference a GHSA — N/A

---------

Co-authored-by: Jeremy [KK7GWY] <kk7gwy@aethersdr.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>