Rebuild the Classic template on the engine with A4/Letter and a template harness

Classic now renders through buildRenderModel, computeFrame, the template
and renderCore, with S.No and HSN/SAC columns, GST slots only through
checked Slot components, taxable value, reverse-charge line, cancelled tag,
continuation band and Page n of N. Letter absorbs its height deficit in
elastic gaps. A harness runs 15 fixtures on A4 and Letter plus an item
sweep and asserts zero audit errors, repeated headers, no split blocks,
rupee extraction and deterministic output. The legacy InvoiceDocument is
removed.

src/pdf/testing/rasterize.ts

@@ -38,3 +38,21 @@ export async function inkPixels(
     await task.destroy();
   }
 }
+
+/** PNG of one page (1-based) at `scale`, white background; for visual review only. */
+export async function pagePng(canvasLib: typeof import("@napi-rs/canvas"), bytes: Uint8Array, pageNo: number, scale = 1.5): Promise<Buffer> {
+  const task = getDocument({ data: new Uint8Array(bytes), useSystemFonts: false, disableFontFace: true });
+  const doc = await task.promise;
+  try {
+    const page = await doc.getPage(pageNo);
+    const viewport = page.getViewport({ scale });
+    const canvas = canvasLib.createCanvas(Math.ceil(viewport.width), Math.ceil(viewport.height));
+    const ctx = canvas.getContext("2d");
+    ctx.fillStyle = "#ffffff";
+    ctx.fillRect(0, 0, canvas.width, canvas.height);
+    await page.render({ canvasContext: ctx as never, canvas: canvas as never, viewport }).promise;
+    return canvas.toBuffer("image/png");
+  } finally {
+    await task.destroy();
+  }
+}