Add evidence-based plugin impact report

[mehul0810]

Closes #182.

Summary

• adds a bounded Plugin Impact report that joins active-plugin Site Inventory data with high-confidence Admin Asset Audit evidence
• reports measured admin screens, scripts, and styles without ranking plugins or claiming an observed asset is slow or safe to disable
• labels frontend assets, query contribution, callback timing, and memory contribution as unavailable until dedicated measurement contracts exist
• keeps collection local, private, bounded, and read-only; the report does not start scans or make external requests
• documents the evidence and limitation contract

Validation

• composer lint
• scoped PHPCS for changed PHP files
• composer test — 137 tests, 435 assertions
• npm run test:unit — 37 tests
• npm run lint
• npm run build
• PHPStan across 59 files
• git diff --check

Hosted WordPress smoke proof must confirm the desktop/mobile report, responsive layout, and evidence/limitation language before merge.

[github-actions]

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