Add clickable link support (annotations + text-detected URLs)

[kdroidFilter]

Closes #8

Summary

• New common API: PdfLink / PageLinks (bounds in PDF points, uri, destPageIndex), PdfDocument.pageLinks() and public PdfReaderState.pageLinks(pageIndex).
• Links come from two PDFium sources, merged with overlap dedup (annotation wins over a text-detected link under the same rect):
  • link annotations via FPDFLink_Enumerate — URI actions and internal GoTo destinations
  • web links via FPDFLink_LoadWebLinks — URLs and e-mail addresses detected in the page text (PDFium returns e-mails as mailto: URIs)
• PdfPage gains linksEnabled: Boolean = true and onLinkClick: ((PdfLink) -> Boolean)?: an invisible clickable overlay per link rect, hand cursor on desktop, click semantics for accessibility, and correct touch-scroll interplay. External URIs open through LocalUriHandler; onLinkClick returning true consumes the click.
• PdfReader scrolls to the destination page for internal GoTo links and exposes the same linksEnabled / onLinkClick knobs.
• Implemented on all five backends:
  • JVM + Android: nCountPageLinks / nExtractPageLinks in the shared pdfium_jni.cpp; JVM degrades to empty links if a prebuilt native predates the API
  • iOS: cinterop (fpdf_doc.h / fpdf_text.h already exposed by the .def)
  • JS / wasmJs: new links op in pdfium_worker.mjs (the shipped pdfium.wasm already exports FPDFLink_*)
• Example app: internal links jump via jumpToPage; README documents the feature.

Test plan

• smokeTest against a 2-page PDF containing all four link kinds — URI annotation, internal GoTo (dest=1), text-detected URL, text-detected e-mail (mailto:) — all extracted correctly
• Compiles: JVM, JS, wasmJs, Android Kotlin + NDK/CMake, Linux JNI glue, example app
• iOS compile on macOS CI (cinterop code follows the file's existing patterns; cannot build on Linux)
• Rebuild prebuilt JNI glue for macOS/Windows hosts (buildJniMacOs / buildJniWindows) before publishing
• Manual click-through on desktop/web: external link opens browser, e-mail opens mail client, GoTo scrolls the reader