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>

pnpm-lock.yaml

@@ -11,6 +11,9 @@ importers:
       '@citation-evidence/engine':
         specifier: link:../citation-engine
         version: link:../citation-engine
+      '@citation-evidence/evidence-source':
+        specifier: link:../evidence-source
+        version: link:../evidence-source
       jszip:
         specifier: ^3.10.1
         version: 3.10.1