Add searchable and flattened export, history re-export and diagnostics self-test

issueAndArchive keeps an issued invoice even when rendering or archiving
fails (Issued - not archived, with a Render & archive action). The export
menu offers a searchable PDF or a 300 DPI image-only PDF whose metadata is
the invoice number alone; flatten failures offer Retry or searchable export
and never silently lower the resolution. History re-exports the original
as issued by default and warns when a re-render's layout fingerprint
differs. The render client gains a FIFO assemble lane that previews cannot
supersede. A Diagnostics panel runs fixtures in the real worker and
compares against goldens generated from Node.

src/pdf/assemble.ts

@@ -0,0 +1,37 @@
+import { createElement as h, type ReactElement } from "react";
+import { Document, Image, Page } from "@react-pdf/renderer";
+
+export interface AssemblePage {
+  /** A baseline JPEG; react-pdf embeds it as is (DCTDecode passthrough, no re-encoding). */
+  jpeg: Uint8Array;
+  widthPt: number;
+  heightPt: number;
+}
+
+export interface AssembleInput {
+  pages: AssemblePage[];
+  /** The only metadata written: a flattened file must not leak the client name or GSTINs. */
+  title: string;
+}
+
+/**
+ * An image-only document: one full-bleed JPEG per page. No author, subject or keywords, and the
+ * title is whatever the caller passes (the invoice number). The images go in as untyped Blobs because
+ * that is the form react-pdf's browser build resolves without sniffing the type.
+ */
+export function assembleImagePdf({ pages, title }: AssembleInput): ReactElement {
+  return h(
+    Document,
+    { title },
+    ...pages.map((p, i) =>
+      h(
+        Page,
+        { key: i, size: [p.widthPt, p.heightPt] as [number, number], style: { padding: 0 }, wrap: false },
+        h(Image, {
+          src: new Blob([p.jpeg as BlobPart]) as never,
+          style: { width: p.widthPt, height: p.heightPt },
+        }),
+      ),
+    ),
+  );
+}