Round 2 P2: timeoutOf() accepted ANY TimeoutReason, so under nesting — when
the upstream handed to deadline() is itself a deadline (the RFC's named
tools/execute middleware follow-up) and its outer timer fires first —
AbortSignal.any preserves the outer reason and the inner bash/web would report
the outer timeout as their own (timedOut / WEB_FETCH_TIMEOUT) though their local
timer never expired. Add an optional code to timeoutOf; bash and web pass their
own code, so a foreign timeout falls through to the upstream-cancel path.
Dudu-0223
760bc9aa6a