Fix waterfall misalignment with spectrum display

[Copilot]

PR #1464 corrected spectrum alignment by replicating the Nyquist sample and drawing m_nbBins+1 points instead of m_nbBins, but only applied this to the spectrum histogram. The waterfall and 3D spectrogram still used m_nbBins pixels, causing rightward shift visible at low FFT sizes.

Changes

• Buffer allocation: Changed waterfall and 3D spectrogram buffer width from m_nbBins to m_nbBins+1
• Data population: Replicate Nyquist sample (spectrum[0]) at end of waterfall and 3D spectrogram lines
• Texture uploads: Updated all subTexture() calls to use m_nbBins+1 width parameter

// In updateWaterfall() and update3DSpectrogram()
for (int i = 0; i < m_nbBins; i++) {
    // ... write spectrum[i] ...
}
// Replicate Nyquist sample to end of positive side
*pix++ = /* spectrum[0] value */;

This brings waterfall rendering in line with the spectrum display, ensuring both use the same bin count and align with the frequency scale.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Spectrum and waterfall misalignment</issue_title>
<issue_description>Sdrangel for MacOS, 7.22.10, MacOS Tahoe 26.0.1, 4k display.

The peak of the spectrum does not align with the peak in the waterfall using the Broadcast FM demod. Sometimes it's slight, sometimes it's quite obvious. Here's a screen shot that shows the problem:

Spectrum/Waterfall Misalignment

The question that arises since they don't agree is: Which should we be using to tune? The spectrum, or the waterfall?

</issue_description>

Comments on the Issue (you are @copilot in this section)

@srcejon Can see it in this image where I've generated a tone at DC, with low FFT size. It's the waterfall that appears shifted to the right.

</comment_new>
<comment_new>@srcejon
Possibly related to this fix I did a while back: #1464 - I only changed the spectrum.

• Fixes Spectrum and waterfall misalignment #2619

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[sonarqubecloud]

Quality Gate passed

Issues
3 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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