feat: expose PDF page indirect object number via EPDF_GetPageObjNum

[mpogodin-readdle]

Summary

PDF pages have a stable indirect object number in the PDF file structure (/Type /Page dictionary GetObjNum()). This number is stable across page reordering, insertion, and deletion — unlike the page index, which shifts whenever pages are moved. This PR exposes that number through the @embedpdf stack so consumers can use it as a permanent, stable page reference.

Use case: features like annotations and AI citations that must survive page reordering. The page index changes when pages are moved; the PDF object number does not.

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Changes

@embedpdf/pdfium

• Added EPDF_GetPageObjNum(document, page_index) → int C++ wrapper in build/code/cpp/main.cpp
  • Uses CPDFDocumentFromFPDFDocument → GetPageDictionary → GetObjNum() from internal PDFium APIs
  • Returns -1 if the document or page dictionary is invalid
• Added EPDF_GetPageObjNum prototype to build/code/cpp/ext_api.h (picked up by the AST export generator)
• Added -I.../pdfium-src to compile.esm.sh and compile.sh to allow internal PDFium header access from main.cpp
• Registered EPDF_GetPageObjNum binding in src/vendor/functions.ts, pdfium.js, and pdfium.cjs

@embedpdf/models

• Added objectNumber?: number to PdfPageObject with JSDoc explaining stability semantics

@embedpdf/engines

• In openDocumentBuffer, calls EPDF_GetPageObjNum after rotation and spreads objectNumber onto each PdfPageObject
• The call is runtime-guarded with typeof epdfGetPageObjNum === 'function' — existing builds without the new WASM export continue working unaffected (objectNumber is simply undefined)

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WASM rebuild required

⚠️ I don't have access to a Docker/Emscripten environment, so I was not able to provide a rebuilt pdfium.wasm binary as part of this PR.

All the C++ source (main.cpp, ext_api.h) and build script changes (compile.esm.sh, compile.sh) are in place. Please run make build as part of merging to compile EPDF_GetPageObjNum into the binary and regenerate the vendor files. The JS/TS bindings are pre-wired and will activate automatically once the WASM is rebuilt.

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Backward Compatibility

No breaking changes. The objectNumber field is optional on PdfPageObject. Older WASM builds silently skip the call and leave the field absent. Consumers must check for undefined before using it.

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Testing

• @embedpdf/models and @embedpdf/pdfium build cleanly
• TypeScript type-check on @embedpdf/engines produces no new errors
• Runtime guard verified to match the existing pattern used by EPDF_GetPageRotationByIndex

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