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/engine/ids.ts

@@ -0,0 +1,19 @@
+/**
+ * Ids the audit recognises. react-pdf copies `id` into the layout tree, so these survive to
+ * auditLayout without any custom prop.
+ */
+
+/** Marks decor (absolute decorative boxes). The audit ignores a decor node and everything under it. */
+export const DECOR_ID = "decor";
+
+/** The continuation band text: legitimately draws 0 lines on page 1. */
+export const BAND_ID = "chrome-band";
+
+/** The "Page n of N" label. */
+export const PAGE_LABEL_ID = "chrome-page";
+
+/** The repeated table header row (fixed). */
+export const TABLE_HEAD_ID = "tbl-head";
+
+/** Body rows are `tbl-row-<index>`: unique, so a row on two pages is visible as a split. */
+export const TABLE_ROW_PREFIX = "tbl-row-";