feat: the invoice PDF, with stamps and each payment (P4b S3)

ledger-import invoice_pdf (templates/invoice.typ): seller, bill-to,
number, dates, terms, lines, total, each settled line and the balance
due; money formatted in Rust (the template does no arithmetic), text as
data, deterministic bytes. Stamps as a rubber stamp would mark it, in
red: PAID (with the day it was paid in full: the invoice is then the
customer's receipt), PARTIALLY PAID, PAST DUE, VOID; DRAFT in gray for
S4's preview. ledger-package invoice_document builds it from the books
as of a day: the derived figures (B-38), each payment that stands on its
own line with its date, payment method and reference; re-created copies
say so (the issued copy is kept from S4 on and never changes).
books.document pdf hands it over as a download; the viewer's invoice
page saves or opens it. The owner asked for the paid stamp with the
payment's date and details (2026-10-24).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T3gXMVt7XJPi6ens1or9Dj

crates/ledger-import/src/lib.rs

@@ -8,6 +8,7 @@
 //! - [`pdf`]: generate PDF financial statements (CPA handoff).
 //! - [`asset_register`]: the fixed asset register as XLSX and PDF.
 //! - [`labels`]: print asset label sheets with QR codes.
+//! - [`invoice_pdf`](mod@invoice_pdf): an invoice as PDF.
 //! - [`fonts`]: the bundled fonts the PDFs and the viewer share.
 //!
 //! All money stays `Decimal`; no floats in money paths.
@@ -17,6 +18,7 @@ pub mod bank_csv;
 pub mod chart_csv;
 pub mod fonts;
 pub mod iif;
+pub mod invoice_pdf;
 pub mod labels;
 pub mod pdf;
 pub mod report;
@@ -30,6 +32,7 @@ pub use iif::{
     Iif, IifAccount, IifChange, IifClass, IifError, IifField, LeftOut, MEMO_MAX, NAME_MAX,
     NUMBER_MAX, QbAccountType, iif_accounts, iif_classes, journal_to_iif,
 };
+pub use invoice_pdf::{InvoiceDoc, InvoiceItem, InvoiceParty, InvoiceStamp, invoice_pdf};
 pub use labels::{AssetLabel, LabelSheet, asset_labels_pdf};
 pub use pdf::{PdfError, statement_pdf};
 pub use report::{Member, ReportMeta, Row, RowKind, Statement, YearEarnings, format_amount};