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>

tests/integration/forms-overlay-e2e.dom.test.tsx

@@ -29,7 +29,7 @@ import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 
 import type { Selector } from "@shared/selector";
 
-import type { PdfSelectionCapture } from "@anchor/index";
+import type { PdfSelectionCapture } from "@citation-evidence/evidence-anchor";
 import manifest from "../../fixtures/pdfs/manifest.json" with { type: "json" };
 
 interface ViewerProps {
@@ -60,8 +60,8 @@ function fakeHighlightRect(): DOMRect {
   } as DOMRect;
 }
 
-vi.mock("@anchor/index", async (importOriginal) => {
-  const original = await importOriginal<typeof import("@anchor/index")>();
+vi.mock("@citation-evidence/evidence-anchor", async (importOriginal) => {
+  const original = await importOriginal<typeof import("@citation-evidence/evidence-anchor")>();
   const MockPdfSpikeViewer = (props: ViewerProps) => {
     viewerSnapshot.pdfUrl = props.pdfUrl;
     viewerSnapshot.scrollToAnnotationId = props.scrollToAnnotationId ?? null;