Add README for scratch-core

[laurensWe]

Adds packages/scratch-core/README.md as an entry point for developers working with or integrating against the core library.

Features:

• Repository context — diagram showing where scratch-core fits in the full request flow (scan upload → preprocessing → scoring → LR), and how it relates to the FastAPI service in src/.
• Package structure — annotated directory tree of all modules.
• Data model — overview of ScanImage, Mark, and the full MarkType enum (striation and impression variants).
• Main pipelines — step-by-step description of both preprocessing pipelines and their comparison counterparts (correlate_profiles / compare_surfaces), including the output fields returned by each.

[vergep]

Nicely done!

[github-actions]

Diff Coverage

Diff: origin/main..HEAD, staged and unstaged changes

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[github-actions]

Package	Line Rate	Branch Rate	Health
.	96%	92%	✔
computations	85%	50%	✔
container_models	99%	100%	✔
conversion	96%	89%	✔
conversion.export	100%	100%	✔
conversion.filter	97%	89%	✔
conversion.leveling	100%	100%	✔
conversion.leveling.solver	100%	75%	✔
conversion.plots	99%	88%	✔
conversion.preprocess_impression	99%	91%	✔
conversion.preprocess_striation	90%	62%	✔
conversion.profile_correlator	96%	82%	✔
conversion.surface_comparison	99%	89%	✔
conversion.surface_comparison.cell_registration	100%	90%	✔
extractors	97%	75%	✔
mutations	100%	100%	✔
parsers	97%	50%	✔
parsers.patches	89%	60%	✔
preprocessors	99%	91%	✔
processors	100%	83%	✔
renders	99%	50%	✔
utils	71%	100%	➖
Summary	98% (3265 / 3338)	86% (335 / 388)	✔

Minimum allowed line rate is 50%