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/docs.ts

@@ -0,0 +1,54 @@
+import { cloneElement, createElement as h, type ReactElement, type ReactNode } from "react";
+import { Document, Page, renderToBuffer } from "@react-pdf/renderer";
+import { computeFrame, PAGE_SIZES, type PageFrame } from "../engine/geometry";
+import { walk, type LayoutNode, type LayoutPage } from "../engine/layoutTree";
+import { stack } from "../fonts/register";
+
+/** Small fixtures for tests that need a tiny react-pdf document. */
+export const FAMILY = stack("IBM Plex Sans");
+export const PAGE_W = 400;
+export const PAD = 20;
+
+export const txt = (extra: Record<string, unknown> = {}) => ({ fontFamily: FAMILY, fontSize: 10, lineHeight: "12pt", ...extra });
+
+export function doc(children: ReactNode, o: { height?: number; width?: number; pad?: number | Record<string, number> } = {}): ReactElement {
+  const pad = o.pad ?? PAD;
+  return h(
+    Document,
+    null,
+    h(
+      Page,
+      { size: { width: o.width ?? PAGE_W, height: o.height ?? 300 }, style: typeof pad === "number" ? { padding: pad } : pad },
+      children,
+    ),
+  );
+}
+
+/** A frame matching `doc()`: uniform padding `PAD`, optional footer reserve inside the bottom padding. */
+export function frameFor(o: { height?: number; width?: number; footerReserve?: number; continuationReserve?: number } = {}): PageFrame {
+  return computeFrame({
+    page: { ...PAGE_SIZES.A4, width: o.width ?? PAGE_W, height: o.height ?? 300 },
+    margins: { top: PAD, right: PAD, bottom: PAD, left: PAD },
+    footerReserve: o.footerReserve ?? 0,
+    continuationReserve: o.continuationReserve ?? 0,
+    letterMode: false,
+  });
+}
+
+export function find(page: LayoutPage, pred: (n: LayoutNode) => boolean): LayoutNode[] {
+  const out: LayoutNode[] = [];
+  walk(page.nodes, (n) => {
+    if (pred(n)) out.push(n);
+  });
+  return out;
+}
+
+export const byId = (page: LayoutPage, id: string) => find(page, (n) => n.id === id);
+
+/** react-pdf's own layout tree, for facts the normalised tree hides (style values, raw props). */
+export async function rawLayout(el: ReactElement): Promise<{ type: string; children: any[] }> {
+  let raw: any;
+  const withHook = cloneElement(el as ReactElement<any>, { onRender: (p: any) => (raw = p._INTERNAL__LAYOUT__DATA_) });
+  await renderToBuffer(withHook as never);
+  return raw;
+}