Embedpdf/feature/digital signature - #17

Introduce FixPageFieldsFromDict(CPDF_Dictionary*) to register orphan widget annotations from a page dictionary and refactor FixPageFields to call it. Expose a new public API EPDF_FixPageFieldsRaw to allow registering page Annots into the interactive form without loading the full page (validate document/page index and page dict). Add EPDFSig_GetAnnotSignatureHandle to obtain an FPDF_SIGNATURE handle from a signature widget annotation and expose its declaration in the public header. Small defensive checks added where appropriate.