Create a new V8 isolate with an optional heap limit in MB. None applies the
bounded-by-default cap (DEFAULT_HEAP_LIMIT_MB) — an isolate is NEVER created
with an unbounded heap, so a guest heap bomb terminates its own isolate rather
than fatal-aborting the shared process.
Register the near-heap-limit OOM guard on an isolate that was created with a
configured heap cap. Without this guard, V8 fatal-aborts the whole (process-
global) runtime with a SIGTRAP when the cap is reached, taking down every
concurrent tenant; with it, the offending isolate is terminated instead.