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>

tests/integration/anchor-source-roundtrip.test.ts

@@ -17,7 +17,7 @@ import { createRequire } from "node:module";
 import { fileURLToPath } from "node:url";
 import { beforeAll, describe, expect, it } from "vitest";
 
-import { ingestPdf } from "@source/pdf/ingest";
+import { ingestPdf } from "@citation-evidence/evidence-source";
 import { createSelectors, resolveSelectors } from "@anchor/selectors";
 import type { PdfSelectionCapture } from "@anchor/types";
 import manifest from "../../fixtures/pdfs/manifest.json" with { type: "json" };