refactor(source): consume extracted evidence-source package (ESRC-WP-0001-T06)

Replace the local src/source PDF slice with the standalone
@citation-evidence/evidence-source package (link:../evidence-source).

- src/source/index.ts is now a thin façade re-exporting the ingest core
  and browser upload helpers from the package
- viewer-url stays local (viewer concern) but takes PdfByteStore from
  the package
- extracted ingest/extract/fingerprint/byte-store/upload files removed
- anchor-source round-trip integration test now exercises the package
  across the repo boundary

Full suite (125 tests), typecheck, and lint all green.

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

package.json

@@ -20,6 +20,7 @@
   },
   "dependencies": {
     "@citation-evidence/engine": "link:../citation-engine",
+    "@citation-evidence/evidence-source": "link:../evidence-source",
     "jszip": "^3.10.1",
     "pdfjs-dist": "^4.4.168",
     "react": "^18.3.1",