fix(ci3): run dedicated benches on bare metal to cut variance

The dedicated bench box used m6a.32xlarge, which is only ~2/3 of a physical
host and shares it with other tenants. A memory-heavy co-tenant steals memory
bandwidth, which showed up as bimodal variance in the bandwidth-bound proving
phases (e.g. avm_bulk wire commitments flipping ~2.1s vs ~3.2s) while compute
phases (sumcheck, trace gen) stayed flat. A controlled experiment confirmed it's
not contention from our own concurrent benches, not NUMA locality, and not code.

Switch to m6a.metal: sole-tenant bare metal, no co-tenant to contend for memory
bandwidth. Still spot-first with on-demand fallback (same sole-tenant hardware
either way), and 96 physical cores gives more bench parallelism than the 32xl.

Author: charlielye

ci.sh

@@ -180,17 +180,21 @@ case "$cmd" in
     ;;
   bench)
     # Launched by the build instance on uploadable runs to produce stable benchmark
-    # numbers on a dedicated instance of a FIXED type. AWS_INSTANCE pins the exact type
-    # (bypasses spot pool diversification) — that's what keeps numbers comparable. Spot
-    # vs on-demand is the same hardware, so we try spot first and fall back to on-demand
-    # (the default fleet behaviour); a mid-run spot reclaim is handled by bootstrap_ec2's
-    # internal on-demand retry. CI_DASHBOARD and PARENT_LOG_ID are inherited from the
-    # launching run so it nests as a sibling job.
+    # numbers on a dedicated, FIXED-type instance. AWS_INSTANCE pins the exact type
+    # (bypasses spot pool diversification) — that's what keeps numbers comparable. We use
+    # bare metal (m6a.metal): a sub-host instance (e.g. m6a.32xlarge) shares its physical
+    # host with other tenants, and a memory-heavy neighbour steals memory bandwidth, which
+    # showed up as bimodal variance in the bandwidth-bound proving phases (MSM commitments)
+    # while compute phases stayed flat. Metal is sole-tenant, so there's no neighbour.
+    # Spot vs on-demand is the same hardware, so we try spot first and fall back to
+    # on-demand (the default fleet behaviour); a mid-run spot reclaim is handled by
+    # bootstrap_ec2's internal on-demand retry. CI_DASHBOARD and PARENT_LOG_ID are
+    # inherited from the launching run so it nests as a sibling job.
     # Timestamp the instance-request output. pipefail (in a subshell, since ci.sh doesn't
     # set it globally) keeps bootstrap_ec2's exit code through add_timestamps — the
     # launching build instance waits on this fatally.
     ( set -o pipefail
-      AWS_INSTANCE=m6a.32xlarge JOB_ID=x-bench INSTANCE_POSTFIX=x-bench \
+      AWS_INSTANCE=m6a.metal JOB_ID=x-bench INSTANCE_POSTFIX=x-bench \
         bootstrap_ec2 "./bootstrap.sh ci-bench" 2>&1 | add_timestamps )
     ;;
   socket-fix)