Add opt-in "snap loops to zero-crossings" processing option

[douglas-carmichael]

Summary

Adds an opt-in Snap loops to zero-crossings processing option.

Some sample libraries ship forward loops whose start and end land on non-matching sample values, so the loop audibly clicks at the wrap-around on every repeat. This is common with auto-sampled instruments whose loop was not designed to be click-free — they rely on the host applying a cross-fade at play time. When the destination format has no loop cross-fade (and none is baked), that click is exported as-is.

This option moves both loop boundaries to a nearby rising zero-crossing so the loop end and start meet near zero, removing the click without resampling the audio — only the stored loop positions change. It complements the existing Set fixed loop-crossfade option (which bakes a cross-fade) for the cases where you would rather not alter the audio at all.

How it works

A new core/algorithm/LoopZeroSnapper decodes each zone to a mono mix and, for every forward loop, searches a small window around the existing start and end for the nearest rising zero-crossing. It is deliberately conservative:

• only forward loops are touched;
• very short (single-cycle) loops are skipped so their pitch is unchanged (< 4096 frames);
• a boundary is moved by at most an eighth of the loop length, capped at 512 frames;
• the new boundaries are applied only when they actually reduce the discontinuity at the wrap;
• a loop end of -1 ("loop to sample end") is materialised to an explicit zero-crossing only when snapping helps.

It runs after the existing resample / bit-depth stage in ConverterBackend.processSamples, so it always sees the final audio.

Wiring

• New Snap loops to zero-crossings check-box in the processing dialog (ProcessingDialog / MainFrame), persisted like the other processing options.
• New CLI flag -Zs (gated by -Ze, like the others).
• DetectSettings.snapLoopsToZero + needsProcessing() gate.
• Off by default.

Example

A real 12-zone Synthstrom Deluge pad whose loops wrap with an audible step. The numbers below are the absolute sample-value jump at each loop wrap, measured on the source audio, before and after enabling the option (16-bit samples, full scale 32768):

zone	jump before	jump after
D#3	381	19
F#3	448	21
A3	405	81
C4	223	223
D#4	382	174
F#4	872	226
A4	825	285
C5	328	169
D#5	1517	1
F#5	1747	0
A5	1398	567
C6	85	1
total	8611	1767

11 of 12 loops improved (−79% total); the C4 zone is left exactly as it was, because no nearby crossing reduced its jump — the conservative guard in action. No loop is ever made worse.

Notes

Happy to adjust the naming, the window / threshold constants, or move the algorithm elsewhere if you'd prefer.