fix(copyright): recover bounded author action credits

[mstykow]

Summary

• recognize action-backed credits inside bounded POD author sections, including rewritten by, maintained by, earlier-work, and help cues
• support explicit author-section heading variants such as AUTHORS and MAINTENANCE and Author and Modification History
• retain explicit contactless POD attributions through the scanner source-code guard and collapse only overlapping duplicate author spans

Issues

• Covers: False-positives when comparing provenant with scancode-toolkit output #1391

Scope and exclusions

• Included: structurally bounded POD author sections and explicit attribution cues
• Explicit exclusions: free-form action prose outside author sections and uncertain attribution text

How to verify

• Scan Perl 5.38.4 with --copyright: 42 source-backed authors are added across 35 files, with no removals; examples include Jim Miner, Andy Lester, Mark Biggar, Tels, the Perl Toolchain Gang, and the Perl 5 Porters.
• Scan InfoZIP 3.0 with --copyright: author, copyright, and holder output is unchanged.

Intentional differences from Python

• This does not reproduce ScanCode's token rendering or its prose false positives. It uses ScanCode deltas to identify missing evidence, then recovers additional valid credits that ScanCode also misses, while preserving Provenant's bounded scanner contract.

Follow-up work

• Created or intentionally deferred: none for bounded POD author action credits.

[mstykow]

@greptileai review