Add pagination blocks, renderCore, layout audit and react-pdf quirk tests

TableGuard, BottomSpacer, FirstPageHeader and PageChrome implement the
verified pagination rules; renderCore returns bytes, a normalised layout
tree, audit issues and a layout fingerprint. auditLayout detects overlaps,
ink overflow, truncation, header-only pages and split blocks. Quirk tests
pin the react-pdf behaviours the design depends on, and a child-process
test pins the page-level absolute Svg hang.

src/pdf/testing/rasterize.ts

@@ -0,0 +1,40 @@
+import { getDocument } from "pdfjs-dist/legacy/build/pdf.mjs";
+
+/** `@napi-rs/canvas` is an optional dependency; null when its native binary is not installed. */
+export async function loadCanvas(): Promise<typeof import("@napi-rs/canvas") | null> {
+  try {
+    return await import("@napi-rs/canvas");
+  } catch {
+    return null;
+  }
+}
+
+/** Number of non-white pixels in the box (x0,y0,x1,y1 in pt from the top-left) of page 1 drawn at `scale`. */
+export async function inkPixels(
+  canvasLib: typeof import("@napi-rs/canvas"),
+  bytes: Uint8Array,
+  box: { x0: number; y0: number; x1: number; y1: number },
+  scale = 2,
+): Promise<number> {
+  const task = getDocument({ data: new Uint8Array(bytes), useSystemFonts: false, disableFontFace: true });
+  const doc = await task.promise;
+  try {
+    const page = await doc.getPage(1);
+    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;
+    const x = Math.floor(box.x0 * scale);
+    const y = Math.floor(box.y0 * scale);
+    const w = Math.ceil((box.x1 - box.x0) * scale);
+    const h = Math.ceil((box.y1 - box.y0) * scale);
+    const data = ctx.getImageData(x, y, w, h).data;
+    let ink = 0;
+    for (let i = 0; i < data.length; i += 4) if (data[i] < 250 || data[i + 1] < 250 || data[i + 2] < 250) ink++;
+    return ink;
+  } finally {
+    await task.destroy();
+  }
+}