fix(arch): verify Arch Linux compare targets

[mstykow]

Summary

• Verify Arch Linux row 22 end-to-end with compare-outputs --profile common on archlinux/packaging/packages/pacman, archlinux/packaging/packages/grep, and the local .PKGINFO sample at testdata/arch/pkginfo/basic.
• Fix shared maintainer author extraction so maintainer-style comments keep the person identity without a synthetic Maintainers prefix.
• Record the verified Arch runs in docs/BENCHMARKS.md, regenerate docs/benchmarks/scan-duration-vs-files.svg, and mark row 22 as verified in the scorecard.
• Compare artifacts reviewed:
  • .provenant/compare-runs/20260423T104715Z-pacman-79411
  • .provenant/compare-runs/20260423T105234Z-grep-86472
  • .provenant/compare-runs/20260423T105313Z-basic-87766

Issues

• Covers: parser verification scorecard row 22 (Arch Linux)

Scope and exclusions

• Included:
  • Arch source-package verification for pacman and grep
  • Arch built-package verification for .PKGINFO
  • Shared maintainer author normalization used by PKGBUILD-style comment lines
  • Benchmark row and chart regeneration for the recorded Arch runs
• Explicit exclusions:
  • No new PKGBUILD parser surface beyond the existing compare-driven verification work
  • No unrelated benchmark backfills or scorecard rewrites outside row 22

Intentional differences from Python

• On archlinux/packaging/packages/grep, Provenant keeps the real PKGBUILD maintainer spelling Sébastien Luttringer <seblu@archlinux.org> instead of degrading it to ASCII.
• On the same PKGBUILD, Provenant preserves the trailing slash in https://www.gnu.org/software/grep/ because that is the literal file text.
• Provenant keeps GPL-3.0-or-later on that PKGBUILD and rejects ScanCode's extra LGPL-2.0-or-later, which comes from a low-coverage 16.67% sequence match spanning lines 10-11 rather than a real second declared license.

Expected-output fixture changes

• Files changed: none
• Why the new expected output is correct: no parser .expected.json or golden YAML fixtures changed; cargo run --manifest-path xtask/Cargo.toml --bin update-copyright-golden -- authors --list-mismatches --show-diff --filter maintainer reported would update 0 file(s) after the maintainer normalization fix.