Variable CACHE_TIERS`Const`

CACHE_TIERS: readonly [
    "HIT",
    "MISS",
    "RENDER",
    "ASSEMBLED",
    "VERIFY",
    "EDGE",
    "MEM",
    "KV",
    "PLAN",
    "DENY",
    "STALE",
    "AGED",
    "REFUSED",
    "ERROR",
] = ...

Every value x-cfw-cache may carry, in one place.

Its own module so a spec can import it without pulling site-do.ts, which instantiates the interpreter at module scope. The e2e assertion drifted against a hand-written list twice.