P5: arm64 loadgen federation — drive arm64 from arm64 (fidelity + parallelism)

[FumingPower3925]

Part of #151.

Problem. loadgen is amd64-only; both arches are load-driven from the amd64 client (msa2-client). Driving the arm64 server (msr1) from an amd64 client over the fabric is fine for throughput but mixes arch characteristics and serializes the two arches (×2 wall, hurts the 24h budget).

Goal. Symmetric load generation per arch for fidelity + parallelism.

Scope.

• Build/stage an arm64 loadgen binary.
• Use loadgen's federation (or run two independent loadgen instances + merge HdrHistograms via report.mergeHistograms) so arm64 server is driven by arm64 and amd64 by amd64.
• loadgen federation is currently 2-node single-peer; assess whether it suffices or whether independent-runs-then-merge is simpler.
• Enables running both arches concurrently → roughly halves wall-clock.

Acceptance. arm64 results produced with an arm64 load driver; both arches run concurrently within budget.

Refs: mage_cluster.go:129-132 (v1.5 backlog note), loadgen federation.go, report/aggregate.go (mergeHistograms).