feat(FN-297): add social proof impression tracking to landing page testimonials (+2 more)

Commits merged:
- docs(FN-297): update FN-275 audit with social proof event locations
- fix(FN-297): format IntersectionObserver mock in test file
- feat(FN-297): add social proof impression tracking to landing page testimonials

Files changed:
apps/web/src/routes/__tests__/index.test.tsx |   7 ++
 apps/web/src/routes/index.tsx                | 111 +++++++++++++++++++++++++--
 docs/product/funnel-audit-p1-cro-2026-05.md  |  19 ++++-
 3 files changed, 127 insertions(+), 10 deletions(-)

Fusion-Task-Id: FN-297

apps/web/src/routes/__tests__/index.test.tsx

@@ -71,6 +71,13 @@ import { useAuthStore } from "@/stores/auth.store";
 
 beforeEach(() => {
   vi.clearAllMocks();
+  vi.stubGlobal(
+    "IntersectionObserver",
+    vi.fn().mockImplementation(() => ({
+      observe: vi.fn(),
+      disconnect: vi.fn(),
+    })),
+  );
 });
 
 test("captures vin_decode_success on successful decode", async () => {