ci: add PHPStan L9 + PHPCS PSR12 + coverage (S1/S2)

[detain]

Summary

Implements S1 (CI gap — no static analysis) and S2 (coverage gap) for phlix-plugin-example.

Changes

• PHPStan L9: phpstan/phpstan dev dependency + phpstan.neon (level 9, include src/)
• PHPCS PSR12: squizlabs/php_codesniffer dev dependency + phpcs.xml (PSR12, include src/, exclude vendor/)
• Coverage: phpunit.xml updated with Cobertura XML output to coverage.xml
• CI Matrix: Updated .github/workflows/test.yml — PHP 8.3 + 8.4 matrix, pcov coverage driver, PHPStan + PHPCS steps after PHPUnit
• Composer scripts: "phpstan": "vendor/bin/phpstan analyse src --level=9" and "phpcs": "vendor/bin/phpcs src --standard=PSR12"
• Bugfix: Removed unused $container property that PHPStan L9 flagged (property.onlyWritten)

Gate Output

$ vendor/bin/phpstan analyse src --level=9
[OK] No errors

$ vendor/bin/phpcs src --standard=PSR12
(no output — clean)

$ vendor/bin/phpunit --colors=always
OK (4 tests, 4 assertions)
Generating code coverage report in Cobertura XML format ... done

Success Conditions

• vendor/bin/phpstan analyse src --level=9 clean
• vendor/bin/phpcs src --standard=PSR12 clean
• vendor/bin/phpunit green
• CI passes on PHP 8.3 + 8.4
• Coverage gate active

---

Finding(s): S1 (CI gap — no static analysis), S2 (coverage gap)
Dependencies: B3 done separately in phlix-server
Consumer impact: None — devinfra only

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Up to standards ✅

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