feat(pdf): extract standalone PDF ingest package (ESRC-WP-0001)

Bootstrap evidence-source from the citation-evidence src/source PDF slice.

- Headless core (src/pdf): ingest/extract/fingerprint, importing domain
  contracts from @citation-evidence/engine/shared (no local copies)
- Browser upload helpers isolated under src/browser behind a ./browser
  entry point, with an eslint boundary keeping the core browser-free
- pnpm/TS/vitest/eslint scaffold; 52 tests (contract + determinism)
- Fixtures resolved from the sibling citation-evidence checkout, not
  duplicated (real PII) — see docs/ADR-0002; suites skip when absent
- Boundary + fixture decisions recorded as docs/ADR-0001 / ADR-0002
- README/SCOPE rewritten; capability.infotech.pdf-evidence-ingest
  registered, NO_CAPABILITIES removed
- Follow-on workplans ESRC-WP-0002..0004 queued; ESRC-WP-0001 finished

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

tests/fixtures.ts

@@ -0,0 +1,51 @@
+/**
+ * Fixture corpus access for evidence-source tests.
+ *
+ * The corpus is owned by the sibling `citation-evidence` checkout (ADR-0002);
+ * it is not duplicated into this repo. Resolution order:
+ *   1. `EVIDENCE_SOURCE_FIXTURE_DIR` env var, if set.
+ *   2. `<repo>/../citation-evidence/fixtures/pdfs` (sibling-checkout default).
+ *
+ * When the corpus is unavailable, `fixtureCorpusAvailable()` returns false so
+ * fixture-driven suites can skip cleanly instead of failing.
+ */
+
+import { existsSync, readFileSync } from "node:fs";
+import { dirname, resolve } from "node:path";
+import { fileURLToPath } from "node:url";
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+
+export const FIXTURE_DIR = process.env.EVIDENCE_SOURCE_FIXTURE_DIR
+  ? resolve(process.env.EVIDENCE_SOURCE_FIXTURE_DIR)
+  : resolve(__dirname, "../../citation-evidence/fixtures/pdfs");
+
+export interface Fixture {
+  readonly id: string;
+  readonly filename: string;
+  readonly page_count: number;
+  readonly known_good_quote: string;
+  readonly known_good_quote_page: number;
+}
+
+interface Manifest {
+  readonly fixtures: readonly Fixture[];
+}
+
+export function fixturePath(filename: string): string {
+  return resolve(FIXTURE_DIR, filename);
+}
+
+export function fixtureBytes(filename: string): Uint8Array {
+  return new Uint8Array(readFileSync(fixturePath(filename)));
+}
+
+export function fixtureCorpusAvailable(): boolean {
+  return existsSync(resolve(FIXTURE_DIR, "manifest.json"));
+}
+
+export function loadFixtures(): readonly Fixture[] {
+  const manifestPath = resolve(FIXTURE_DIR, "manifest.json");
+  const manifest = JSON.parse(readFileSync(manifestPath, "utf8")) as Manifest;
+  return manifest.fixtures;
+}