fix(copyright): recover chained action credits

[mstykow]

Summary

• Recover contactless people from explicit trailing action clauses such as modified by NAME.
• Recover the first credited person from bounded subject is by NAME, modified by NAME chains.
• Require person-shaped names so tool and generator subjects remain rejected.

Issues

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

Scope and exclusions

• Included: line-local chained action attribution recovery.
• Explicit exclusions: unrelated source-comment prefixes and collective maintainer attribution, which are separate follow-up layers.

How to verify

• Scan Info-ZIP 3.0 with --copyright; win32/crc_i386.c should include Chr. Spieler, and amiga/stat.c should include both Paul Wells and Paul Kienitz.
• The same before/after corpus scan leaves author output unchanged for Perl 5.38.4, musl, Dancer2, and Valgrind, and leaves copyright and holder output unchanged across all five targets.

Intentional differences from Python

• Provenant additionally recovers Paul Wells from This stat() is by Paul Wells, modified by Paul Kienitz; ScanCode only reports the second person.

Follow-up work

• Created or intentionally deferred: source-comment action credits and contactless collective maintainer credits will be handled in subsequent stacked PRs.

Expected-output fixture changes

• Files changed: none.
• Why the new expected output is correct: focused unit coverage exercises the new positive and negative boundaries.

[mstykow]

@greptileai review