iterator gap fix

[kylmcgr]

• NWBAxisArrayIterator now splits chunks that contain a gap.
• NWBClockDrivenProducer now returns a CoordinateAxis with the true per-sample timestamps instead of a LinearAxis.

[Copilot]

Pull request overview

This PR improves correctness when NWB streams have explicit timestamps that include gaps by preventing time axes from implicitly “flattening” over missing time. It updates both chunked iteration and clock-driven playback to preserve true per-sample time semantics across gaps.

Changes:

• Add gap detection (find_gaps) and use it to split iterator chunks that span timestamp gaps.
• Update NWBSlicer.read_by_time (and NWBClockDrivenProducer) to emit a CoordinateAxis with true per-sample timestamps when a requested window spans a gap.
• Add comprehensive tests + new gappy/irregular NWB fixtures to validate behavior, including the gap_tol control.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/ezmsg/nwb/slicer.py	Adds gap detection helper and switches read_by_time to CoordinateAxis when windows span gaps.
src/ezmsg/nwb/iterator.py	Splits chunk messages at detected timestamp gaps; plumbs gap_tol through iterator settings and prefetch worker.
src/ezmsg/nwb/clockdriven.py	Forwards gap_tol into slicer time-window reads so clock-driven playback preserves true timestamps across gaps.
tests/conftest.py	Adds session-scoped fixtures that generate synthetic gappy and irregular timestamped NWB files for testing.
tests/test_slicer.py	Adds slicer-level tests covering gap-free windows, gap-spanning windows, inside-gap empty windows, and irregular streams.
tests/test_iterator.py	Adds iterator-level tests ensuring chunks are split at gaps without dropping/reordering samples; tests gap_tol behavior.
tests/test_clockdriven.py	Adds clock-driven tests verifying empty chunks inside gaps, CoordinateAxis for gap-spanning windows, and multi-producer sync across gaps.

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