refactor(anchor): consume extracted evidence-anchor package (EANCH-WP-0001-T05)

Replace the in-repo src/anchor/ slice with a dependency on the extracted
@citation-evidence/evidence-anchor package (link:../evidence-anchor).

- package.json: add the link dependency
- repoint all @anchor/{index,selectors,types} imports and vi.mock targets to
  the package barrel; the node-env anchor-source-roundtrip test uses the pure
  ./selectors and ./types subpaths (no pdfjs pulled into a node environment)
- drop the @anchor alias from tsconfig paths and vite resolve
- delete src/anchor/ (behavior now owned by evidence-anchor)

Verified green: pnpm typecheck, pnpm test (26 files / 95 tests), pnpm build.

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

pnpm-lock.yaml

@@ -11,6 +11,9 @@ importers:
       '@citation-evidence/engine':
         specifier: link:../citation-engine
         version: link:../citation-engine
+      '@citation-evidence/evidence-anchor':
+        specifier: link:../evidence-anchor
+        version: link:../evidence-anchor
       '@citation-evidence/evidence-source':
         specifier: link:../evidence-source
         version: link:../evidence-source