MIDI Pick-up Mode: the rapid-movement branch is unreachable

**🤖 AI:** Pick-up Mode's rapid-movement handling never executes. [Its help text](https://github.com/jamulussoftware/jamulus/blob/ee40cfe20ecebe92eeec9eecc2b9ee8b948f439f/src/clientsettingsdlg.cpp#L427-L430) promises a control waits until the physical controller matches the software value, and [an inner branch](https://github.com/jamulussoftware/jamulus/blob/ee40cfe20ecebe92eeec9eecc2b9ee8b948f439f/src/audiomixerboard.cpp#L94-L96) exists to also catch a controller swept fast enough that its values *"skip over"* that value. That branch is unreachable, for two independent reasons.

`midiPickupTryApply` [returns before appending](https://github.com/jamulussoftware/jamulus/blob/ee40cfe20ecebe92eeec9eecc2b9ee8b948f439f/src/audiomixerboard.cpp#L119-L129) to the history buffer, so while a control is armed the buffer stays empty and the `size() >= 2` guard in front of the crossing test never holds. Independently, [`prevMidi` reads `back()`](https://github.com/jamulussoftware/jamulus/blob/ee40cfe20ecebe92eeec9eecc2b9ee8b948f439f/src/audiomixerboard.cpp#L97-L99), while [the sole call site](https://github.com/jamulussoftware/jamulus/blob/ee40cfe20ecebe92eeec9eecc2b9ee8b948f439f/src/audiomixerboard.cpp#L113-L119) has already pushed the incoming sample onto that same buffer — so `prevMidi` is the incoming sample, and the bracket test reduces to `midiValue == currentValue`, which the tolerance check above it has already accepted.

Driving those helpers verbatim across every strictly-bracketing two-sample crossing of the fader range, both samples outside the tolerance: on ee40cfe2, 0 of 147,440 pick up. Repairing either defect alone leaves 0. Repairing both gives 147,440. What remains on stock is the tolerance window by itself — armed at fader 50, 5 of 101 controller positions are accepted, 48 through 52 — so a fader swept briskly steps over the window and stops responding, which is the case the unreachable branch was written for.

Present since [the MIDI GUI PR](https://github.com/jamulussoftware/jamulus/pull/3502).

🤖 *This message was written by AI and reviewed by @mcfnord.*

Start in src/audiomixerboard.cpp at midiPickupTryApply, the crossing branch, prevMidi, and their call site; read the Pick-up Mode help text in src/clientsettingsdlg.cpp for intended behavior. Trace history updates and previous-sample selection, then validate that strictly bracketing rapid movements are handled while the existing tolerance window remains correct.

Written by the indexing model from the issue text.