Harden 1.6.0 release readiness

[mehul0810]

Summary

• Finalize 1.6.0 WordPress.org readme metadata, changelog, upgrade notice, and synced POT file.
• Harden Assets Manager admin-bar URL toggling so missing query strings do not trigger notices and existing query args are preserved.
• Sign page-cache regeneration requests so stale refreshes can bypass their own lock without allowing public header spoofing.
• Fix self-pingback filtering to mutate WordPress' by-reference link list, and treat no-change settings saves as successful.
• Make local plugin zip generation install production Composer dependencies in the temporary build directory and exclude PHPUnit cache artifacts.

Validation

• composer validate --strict
• composer lint
• composer test
• composer phpstan
• scoped PHPCS on touched PHP files and tests
• npm run lint
• npm run build
• npm run plugin-zip
• npx playwright test --list
• zip artifact exclusion check for dev/test/source paths
• Studio WP plugin status/version smoke

Notes

• Full repository-wide composer check-cs still reports existing style/tooling debt outside this focused readiness patch; touched PHP files pass scoped PHPCS.

Refs #53

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