finance_report.rs renders TripReceipts into a self-contained PDF with the
nigig PDF stack (nigig-pdf-graphics, base-14 Helvetica): a summary table of
dates and amounts with a total row, then one receipt block per trip, A4
with pagination and a bookmark outline. build_trip_report is pure and its
tests read the output back through PdfDocument, asserting page sizes,
dates, amounts and the total landed. Native-only (gated behind
not(wasm32)), so the wasm build stays free of the PDF dependency. 6 tests,
plus a runnable example.
