EANCH-WP-0001 T04: extract PDF adapter + scroll/highlight helpers

Move pdf-viewer-adapter-spike.tsx, scroll-job.ts (+test), highlight-styles.css
and debug-textlayer.css into src/pdf/ (the adapter boundary). Rewrite the
spike's @shared import to @citation-evidence/engine/shared and its ./types to
../types. Add src/css.d.ts so tsc accepts side-effect .css imports.

Decisions:
- adapter stays the explicitly-named PdfSpikeViewer spike; promotion to a
  production PDFViewerAdapter is registered as T06 follow-on work
- viewer libs (pdfjs-dist, react, react-pdf-highlighter-plus) confined to
  src/pdf/ and exposed only via the evidence-anchor/pdf subpath; the root
  entrypoint stays pure. Boundary enforced by eslint no-restricted-imports.
- ported unit suites (pdf-selector-math round-trip + scroll-job retry) serve
  as the inspectable harness for capture->selectors->resolve->scroll.

Verified: pnpm test (30 passed), typecheck, lint all green. README layout updated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

eslint.config.js

@@ -31,9 +31,9 @@ export default tseslint.config(
     },
   },
   {
-    // The pure core (selectors, resolution, highlight helpers) must not pull in
-    // any concrete viewer library — those live only under src/pdf/.
-    files: ["src/selectors/**/*.ts", "src/highlight/**/*.ts", "src/types.ts"],
+    // The pure core (selectors, resolution, adapter-side types) must not pull
+    // in any concrete viewer library — those live only under src/pdf/.
+    files: ["src/selectors/**/*.ts", "src/types.ts"],
     rules: {
       "no-restricted-imports": [
         "error",