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>

tsconfig.json

@@ -27,7 +27,6 @@
     "paths": {
       "@shared/*": ["../citation-engine/src/shared/*"],
       "@engine/*": ["../citation-engine/src/engine/*"],
-      "@anchor/*": ["src/anchor/*"],
       "@source/*": ["src/source/*"],
       "@work/*":   ["src/work/*"],
       "@binder/*": ["src/binder/*"],