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/blocks/Row.tsx

@@ -5,6 +5,8 @@ import { AvailableWidthProvider, useAvailableWidth } from "./AvailableWidth";
 import { pdfFlags, type PdfFlagsInput } from "./pdfProps";
 
 export interface RowProps extends PdfFlagsInput {
+  /** Copied into the layout tree (audits find rows by it). */
+  id?: string;
   /** One spec per child, in order. */
   widths: ColSpec[];
   gap?: number;
@@ -24,7 +26,7 @@ export interface RowProps extends PdfFlagsInput {
  * cell's content width through useAvailableWidth(). When a column falls under its `min` (or the row is
  * narrower than `stackBelow`), the children are stacked at full width instead.
  */
-export function Row({ widths, gap = 0, stackBelow, align = "flex-start", width, style, children, fixed, wrap, minPresenceAhead }: RowProps) {
+export function Row({ id, widths, gap = 0, stackBelow, align = "flex-start", width, style, children, fixed, wrap, minPresenceAhead }: RowProps) {
   const inherited = useAvailableWidth();
   const total = width ?? inherited;
   const kids = Children.toArray(children);
@@ -32,7 +34,7 @@ export function Row({ widths, gap = 0, stackBelow, align = "flex-start", width,
 
   const { widths: w, ok } = colsDetailed(total, widths, gap);
   const stacked = !ok || (stackBelow !== undefined && total < stackBelow);
-  const flags = pdfFlags({ fixed, wrap, minPresenceAhead });
+  const flags = { ...(id !== undefined ? { id } : {}), ...pdfFlags({ fixed, wrap, minPresenceAhead }) };
 
   if (stacked) {
     return (