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/app/main.tsx

@@ -0,0 +1,12 @@
+import { StrictMode } from "react";
+import { createRoot } from "react-dom/client";
+import { SpikeApp } from "./SpikeApp";
+
+const container = document.getElementById("root");
+if (!container) throw new Error("#root not found");
+
+createRoot(container).render(
+  <StrictMode>
+    <SpikeApp />
+  </StrictMode>,
+);