Fetch resource ACL and fallback ACL concurrently using Promise.all
instead of sequentially. This reduces latency when combined with
HTTP/2 multiplexing, as the speculative fallback fetch overlaps
with the resource ACL fetch at minimal cost.

- internal_fetchAcl now issues both fetches in parallel
- Fallback wrapped in .catch(() => null) to prevent regression
  when speculative fetch fails
- Updated tests for parallel fetch behavior

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

The parallel ACL fetching now consumes mock responses that were
previously unused (fallback fetch was skipped when resource ACL
existed). Add a 4th mock response for the saveAclFor step in
the 3 affected "ignores the fallback ACL" tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>