Native crash (SIGILL) when two PdfReaderStates render concurrently on Android #15

Version: dev.nucleusframework:pdfium-android 156.0.8066.0

Setup: Two PdfReaderStates open at the same time, each with a different document, both rendering via PdfPage. In our case that's a HorizontalPager where each page hosts its own reader, so while swiping two pages are composed and rendering at once.

Result: Native crash after a few back-and-forth swipes:

Fatal signal 4 (SIGILL), code 1 (ILL_ILLOPC), fault addr 0x798d366cb8 in tid 17098 (pdfium-worker-2)

Likely cause: openPdfDocument creates a dedicated pdfium-worker-N executor per document (PdfDocument.android.kt), so each document is serialised on its own thread. But PDFium itself is process-wide and not thread-safe (the POOL_SIZE = 1 comment already notes the shared FreeType FT_Library). With two documents open, two worker threads call into PDFium concurrently, which is a different problem from the intra-document case that POOL_SIZE guards against.

Suggested fix: Serialise all PDFium calls across documents, e.g. route render/pageSize/close through the single Pdfium.sharedDispatcher (or a global mutex) rather than a per-document executor.

Repro: Two PdfPages from two rememberPdfReaderState()s in a HorizontalPager, swipe back and forth repeatedly.