feat: support existing PDF signature fields

[vikas-kushwaha-dev]

Summary

Adds typed support for inspecting and selecting existing PDF signature fields using the JSignPdf 3.2 CLI capabilities.

• Add immutable SignatureField model exposing field name, page, coordinates, signed/blank state, hidden state, and rectangle visibility
• Add JSignService::getSignatureFields() and facade support for inspecting existing fields
• Parse the documented --list-sig-fields output, including Unicode names, spaces, hidden fields, zero-size rectangles, and selector-shadow messages
• Add JSignParam::setSignatureField() / getSignatureField() for selecting an existing field during signing
• Pass field names directly to JSignPdf through --sig-field without implementing selector semantics in PHP
• Preserve JSignPdf diagnostics for invalid, missing, or already-signed fields
• Clean up temporary PDFs after successful and failed inspection
• Add unit and integration coverage using real PDF signature fields
• Document inspection and existing-field signing in the README

Testing

• composer cs:check
• composer test:unit
• composer psalm
• composer test:integration

All checks pass.

Closes #62

[vitormattos]

Looks good to me. I checked the implementation against #62, including the parser, signature field selection, error handling, temporary file cleanup and the integration tests.

The implementation follows the JSignPdf 3.2 behavior without adding PDF or selector logic in PHP, and the current CI is passing.