Fix repeated PDF exports accumulating object references

[ac1982]

Repeatedly serializing the same GoPdf appends page and content references again, so a one-page document becomes a two-page PDF on its second export. Protected documents also gain a new encryption object each time.

Rebuild the derived page references during prepare and reuse the existing encryption object. This keeps repeated exports stable while still including pages added after an earlier export.

Fixes #327.

Validation:

• Regression covers plain/protected documents, embedded text, GetBytesPdfReturnErr, WriteTo, WritePdf, retry after a failed destination write, and adding a page between exports. It fails on the original implementation.
• mkdir -p test/out && go test -race -count=1 ./... and go vet ./... pass on Go 1.27.1/Linux arm64.
• External pdfinfo check: export, repeat export, then add a page produces page counts 1/1/2, versus 1/2/4 and syntax errors before the fix.

The existing behavior where WriteTo discards writer errors is unchanged; the retry test checks document stability, not error propagation.

[oneplus1000]

Thanks @ac1982 for your PR! I normally check PRs on weekends. However, I have to go out of town this weekend, so I might reply a bit later than usual.