Covers getCsrfToken (cookie parsing, URL-decoding, server-side null),
withCsrf (header injection, immutability, no-op when absent),
makeCsrfFetch (method filtering, case-insensitivity, inner-vs-global),
and csrfFetch (regression guard: vi.stubGlobal is honoured at call time,
not bypassed by a module-level captured reference).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>