refactor: decompose monolithic MEA pipeline scripts into focused subpackages

[Copilot]

mea_analysis_routine.py (1,783 lines) and run_pipeline_driver.py (385 lines) were single-file monoliths with no internal modularity, making navigation, testing, and extension painful.

New IPNAnalysis/pipeline/ package

Each pipeline phase is now a dedicated mixin; MEAPipeline in core.py assembles them via Python MRO:

Module	Responsibility
stages.py	ProcessingStage enum, NpEncoder, schema version
defaults.py	Default kwarg factory functions
_setup_mixin.py	Logger, runtime controls, metadata parsing
_checkpoint_mixin.py	Checkpoint load / save / skip
_preprocessing_mixin.py	Phase 1: filter chain, binary save
_sorting_mixin.py	Phase 2: Kilosort4 + spike-detection-only path
_merge_mixin.py	Phase 2.5: UnitMatch + SI auto-merge
_analyzer_mixin.py	Phase 3: SortingAnalyzer + extensions
_reports_mixin.py	Phase 4: curation, probe/waveform plots
_burst_mixin.py	Network burst analysis, raster sort
core.py	MEAPipeline (mixin assembly)
options.py	MEARunOptions / MEARunResult frozen dataclasses
runner.py	run_mea_pipeline() + stage-rewind helpers
cli.py	Argument parser + main()

New IPNAnalysis/driver/ package

run_pipeline_driver.py split into file_scanner.py (H5/NWB/RAW discovery), launcher.py (subprocess + logger), and cli.py (argument parser + main()).

Backward compatibility

Both original files are kept as thin re-export facades — zero breaking changes for existing callers:

# Still works unchanged
from mea_analysis_routine import MEAPipeline, run_mea_pipeline, MEARunOptions

setup.py updated to register the two new subpackages.

Bug fix

--unitmatch-oversplit-min-probability CLI help text incorrectly advertised 0.80; corrected to 0.99 to match the actual default in defaults.py.

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