Implement CE-WP-0002 T01-T02: engine types + PDF viewer adapter spike

T01: shared engine types (Document, Selector union, Annotation, EvidenceItem,
branded IDs with newId factory) per wiki/SharedContracts.md §1-§3.

T02: react-pdf-highlighter-plus v1.1.4 spike behind the §5
DocumentViewerAdapter contract in src/anchor/. Pure round-trip math
extracted to pdf-selector-math.ts with 11 unit tests proving lossless
capture → selectors → JSON → restored-rects. ADR-0004 accepted; full
user-flow Playwright verification deferred to T09.

Adds Vite app shell (index.html, src/app/SpikeApp.tsx) so the spike is
exercisable via pnpm dev. tsconfig --noEmit prevents tsc -b from
littering src/ with stray .js outputs.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/anchor/pdf-selector-math.ts

@@ -0,0 +1,79 @@
+/**
+ * Pure, library-free transformations between the adapter's
+ * `PdfSelectionCapture` and the shared `Selector[]` shapes.
+ *
+ * Extracted from `pdf-viewer-adapter-spike.tsx` so the architectural
+ * round-trip contract (capture → selectors → reconstructed rects) can be
+ * unit-tested without pulling in `react-pdf-highlighter-plus`, React, or a
+ * browser. The spike component re-exports `selectorsFromPdfCapture` from
+ * here so there is one implementation, not two.
+ *
+ * This module is the source of truth for T02's "static evidence that the
+ * round-trip is lossless" — see ADR-0004.
+ */
+
+import type {
+  NormalizedRect,
+  PdfRectSelector,
+  Selector,
+  TextQuoteSelector,
+} from "@shared/selector";
+import type { PdfSelectionCapture } from "./types";
+
+/** Build `Selector[]` from a captured PDF selection. */
+export function selectorsFromPdfCapture(capture: PdfSelectionCapture): Selector[] {
+  const out: Selector[] = [];
+  if (capture.text.length > 0) {
+    const textQuote: TextQuoteSelector = {
+      type: "TextQuoteSelector",
+      exact: capture.text,
+    };
+    out.push(textQuote);
+  }
+  if (capture.rects.length > 0) {
+    const rect: PdfRectSelector = {
+      type: "PdfRectSelector",
+      page: capture.page,
+      rects: capture.rects,
+    };
+    out.push(rect);
+  }
+  return out;
+}
+
+/** Find the `PdfRectSelector` in a selector list, if any. */
+export function findPdfRectSelector(
+  selectors: readonly Selector[],
+): PdfRectSelector | null {
+  return (
+    selectors.find((s): s is PdfRectSelector => s.type === "PdfRectSelector") ?? null
+  );
+}
+
+/** Find the `TextQuoteSelector` in a selector list, if any. */
+export function findTextQuoteSelector(
+  selectors: readonly Selector[],
+): TextQuoteSelector | null {
+  return (
+    selectors.find((s): s is TextQuoteSelector => s.type === "TextQuoteSelector") ??
+    null
+  );
+}
+
+/** Bounding rectangle of a non-empty list of normalized rects. */
+export function unionRect(rects: readonly NormalizedRect[]): NormalizedRect | null {
+  if (rects.length === 0) return null;
+  const first = rects[0]!;
+  let minX = first.x;
+  let minY = first.y;
+  let maxX = first.x + first.width;
+  let maxY = first.y + first.height;
+  for (let i = 1; i < rects.length; i++) {
+    const r = rects[i]!;
+    if (r.x < minX) minX = r.x;
+    if (r.y < minY) minY = r.y;
+    if (r.x + r.width > maxX) maxX = r.x + r.width;
+    if (r.y + r.height > maxY) maxY = r.y + r.height;
+  }
+  return { x: minX, y: minY, width: maxX - minX, height: maxY - minY };
+}