Add MergePoints waveform fallback

[yoshihito-saito]

Summary

Add a waveform extraction fallback in calc_CellMetrics/getWaveformsFromDat.m for sessions where basename.dat is missing but subfolder amplifier.dat files are available.

Changes

• keep the existing behavior when basename.dat exists
• fall back to subfolder amplifier.dat files when basename.dat is missing
• require basename.MergePoints.events.mat for the fallback path
• use MergePoints.foldernames and MergePoints.timestamps_samples to map spikes to each segment
• verify sample-count consistency between MergePoints and each segment file
• skip spikes too close to segment boundaries
• set spikes.processinginfo.params.WaveformsSource to MergePoints amplifier.dat files

Why

Some recordings are stored as segmented amplifier.dat files instead of a single top-level basename.dat. This change allows waveform extraction to proceed in those cases while preserving the original behavior for standard sessions.

[yoshihito-saito]

Manual validation was performed on a copied session containing both a top-level basename.dat and segmented subfolder amplifier.dat files.

To make the direct and fallback paths comparable, waveform extraction was tested with nPull set high enough to avoid random spike subsampling.

Results:

• the direct path (basename.dat) and the fallback path (MergePoints + subfolder amplifier.dat) both extracted raw waveforms for all 71 units
• unit ordering matched across paths
• waveform extraction times matched for 70/71 units overall
• after excluding spikes within one waveform window of segment boundaries, waveform extraction times matched for 71/71 units

The fallback path intentionally skips boundary-adjacent spikes, since their waveform window would cross a subfolder file boundary and could otherwise produce incomplete waveforms. The remaining full-comparison mismatch is consistent with that expected behavior.