test(copyright): split detector test buckets

[abraemer]

Summary

• split module-wide detector tests into semantic buckets for author pipeline, multiline repairs, copyright/holder pipeline, and parser internals
• keep broad end-to-end detector coverage in src/copyright/detector/tests.rs while moving narrower clusters into adjacent module-wide files
• prune redundant fixture-backed and parser scaffolding tests that were already covered elsewhere, reducing src/copyright/detector/tests.rs to 123 tests

Issues

• None.

Scope and exclusions

• Scope is limited to test organization under src/copyright/detector/
• No production detector behavior changes
• No golden fixture updates

Intentional differences from Python

• None.

Follow-up work

• If desired, do one final audit of the remaining broad end-to-end tests in src/copyright/detector/tests.rs, but there is no obvious remaining clean semantic bucket

Expected-output fixture changes

• None.