Initial commit: FoFreeXit Phase 1 (Viewer) + Phase 2 (Annotate)

PDF viewer (render/search/text-select/thumbnails/outline) and annotation
engine (Highlight/Underline/Strikeout/Square/FreeText/Note) over PDFium +
lopdf, with Unicode font embedding for Vietnamese text and undo/redo.

corpus/hello.pdf

@@ -0,0 +1,31 @@
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+endobj
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+<< /Length 57 >>
+stream
+BT /F1 24 Tf 72 700 Td (Hello FoFreeXit - Phase 1) Tj ET
+endstream
+endobj
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+<< /Type /Font /Subtype /Type1 /BaseFont /Helvetica >>
+endobj
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+trailer
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+startxref
+417
+%%EOF