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-cli/tests/spec_mcp.rs

@@ -2727,6 +2727,18 @@ fn documents_list_and_timeline_over_the_wire() {
         ],
         "{one}"
     );
+    // Its PDF, by download (P4b S3): re-created from the books.
+    let with_pdf = m.ok(
+        "books.document",
+        json!({"ref": format!("invoice:{late}"), "as_of": "2026-03-15", "pdf": true}),
+    );
+    let bytes = m.wire.fetch(&with_pdf["pdf"]);
+    assert!(bytes.starts_with(b"%PDF"), "{with_pdf}");
+    assert_eq!(with_pdf["pdf"]["file_name"], format!("invoice-{late}.pdf"));
+    assert!(
+        m.tool("books.document", json!({"ref": "bill:1", "pdf": true}))
+            .refused()
+    );
     assert!(
         m.tool("books.document", json!({"ref": "invoice"}))
             .refused()