perf(cli): raise external-memory-max-growing-factor to 8
Once the pooled ArrayBuffer allocator removes allocator-side cost, profiling
an allocation-heavy loop (streaming fetch/file/encoder bodies) shows the
residual time is V8's external-memory GC accounting: V8 only grows the
external-memory limit by external-memory-max-growing-factor (default 1.1)
per GC, so cycling large backing stores keeps the limit pinned near current
usage and drives continuous concurrent marking/sweeping.
Raising the factor to 8 lets the limit jump above churn so far fewer majors
fire. On a touched-bytes 64 KiB byte-stream drain this stacks additively
with the pool: -23% us/chunk / +31% GB/s vs baseline, peak RSS flat.
Closes #35808
