test(kernel-utils): use vi.fn() spies in sheafify e2e tests to verify dispatch

Replace string-log side-channels and return-value inference with explicit
vi.fn() spy assertions. Each section's handler is a named mock; tests call
expect(spy).toHaveBeenCalledWith(...) and .not.toHaveBeenCalled() to verify
routing directly rather than inferring it from coincident return values.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: grypez