fix(copyright): improve zlib verification attribution

[mstykow]

Summary

• improve generic author extraction for plaintext contributor rosters and wrapped written on top of ... by prose, plus trim prose tails after contact-bearing attributions
• verify madler/zlib with compare-outputs --profile common, rerun narrow copyright golden coverage, and record the benchmark snapshot in docs/BENCHMARKS.md
• regenerate docs/benchmarks/scan-duration-vs-files.svg after adding the zlib row

Issues

• Covers: madler/zlib verification work from docs/implementation-plans/package-detection/PARSER_VERIFICATION_SCORECARD.md

Scope and exclusions

• Included:
  • generic copyright/author heuristic fixes in the shared detector pipeline
  • focused regression tests for the new author-extraction paths
  • zlib compare-output verification and benchmark documentation update
• Explicit exclusions:
  • no scorecard status change, because the Autotools row is already marked verified
  • no parser or golden fixture updates were needed

Intentional differences from Python

• Provenant keeps the more specific LicenseRef-scancode-info-zip-2009-01 AND Zlib classification on contrib/minizip/unzip.c instead of collapsing it to plain Zlib

Follow-up work

• Created or intentionally deferred:
  • no new follow-up item created; the remaining compare tail is limited to legacy attribution/doc differences outside this benchmark row’s recorded end-state advantages

Expected-output fixture changes

• Files changed: none
• Why the new expected output is correct: shared detector changes passed test_golden_authors and test_golden_ics, so no owned fixture drift required an update

Verification

• cargo test plaintext_roster_lines_extract_individual_authors --lib
• cargo test written_on_top_of_line_extracts_author --lib
• cargo test refine_author_truncates_trailing_prose_after_contact --lib
• cargo test written_by_author_email_for_project_is_extracted --lib
• cargo test --features golden-tests test_golden_authors --test copyright_golden
• cargo test --features golden-tests test_golden_ics --test copyright_golden
• cargo run --manifest-path xtask/Cargo.toml --bin compare-outputs -- --repo-url https://github.com/madler/zlib.git --repo-ref f9dd6009be3ed32415edf1e89d1bc38380ecb95d --profile common
• cargo run --manifest-path xtask/Cargo.toml --bin generate-benchmark-chart

Compare artifacts

• Final compare run: .provenant/compare-runs/20260501T105917Z-zlib-47314/
• Prior triage run: .provenant/compare-runs/20260501T104122Z-zlib-6507/