v0.0.1 of flutter_readium_platform_interface

Platform interface for the flutter_readium plugin. Provides shared Dart models, the method-channel contract, and the reader API surface used by native implementations. Changelog excerpt: ### Added - PDFPreferences— model for PDF reader display preferences (layout: PDFLayout?, readingProgression: PDFReadingProgression?, pageSpacing: double?, fit: PDFFit?) with toJson/ fromJsonand copyWith. - PDFLayout— enum (paginated/ scrollVertical/ scrollHorizontal) used by PDFPreferences. Unifies iOS's scroll+ scrollAxisand Android's scrollAxisinto a single cross-platform setting. - PDFReadingProgression— enum (ltr/ rtl) used by PDFPreferences. - `PDFF[...]