feat(pdftract-2m3gl): implement PHP SDK with Packagist publishing
- Add jedarden/pdftract Composer package (sdk/php/)
- Implement Client.php with proc_open subprocess execution
- Add PSR-3 LoggerInterface integration (defaults to NullLogger)
- Add 9 contract methods: extract, extractText, extractMarkdown, extractStream, search, getMetadata, hash, classify, verifyReceipt
- Add readonly model classes: Document, Page, Metadata, Fingerprint, Classification, Match, Receipt
- Add exception classes: PdftractException base + 8 subclasses
- Add PHPUnit conformance test suite
- Add phpunit.xml configuration
- Add composer.json with jedarden/pdftract package name
- Add .ci/argo-workflows/pdftract-php-publish.yaml (Packagist auto-discovery from git tags)
Also includes Ruby SDK scaffold from parallel workflow.
Closes pdftract-2m3gl
246befd8d1