feat(benchmark): add -b/--batch flag, test MoE batch size impact

Add batch size override to benchmark scripts. Testing -b 256 vs default
2048 on Vulkan RADV shows no meaningful difference for MoE pp2048
(826 vs 843 t/s, within noise). Community-reported +70% improvement
does not reproduce on this backend.

docs/optimization-log.md

@@ -104,11 +104,14 @@ Living document tracking what was applied, tested, and the actual results. Each
 
 ### 3.2 MoE Batch Size `-b 256`
 
-- **Date**: PENDING
-- **Change**: Add `-b 256` to MoE benchmark runs
-- **Expected**: Up to +70% pp improvement for MoE models (community benchmarks)
-- **Benchmark**: Not yet run
-- **Verdict**: PENDING
+- **Date**: 2026-03-30
+- **Change**: `-b 256` vs default (2048)
+- **Benchmark**: `data/benchmarks/batch-default-*` vs `data/benchmarks/batch-256-*`
+- **Result** (Vulkan RADV, Qwen3.5-35B-A3B UD-Q4_K_XL, q4_0 KV):
+  - Default: 826 pp, 55.9 tg
+  - b=256: 843 pp, 55.5 tg (within noise)
+- **Notes**: Community-reported +70% improvement does not reproduce on Vulkan RADV. May only apply to ROCm or CPU backends, or to longer prompts (pp8192+).
+- **Verdict**: NO IMPACT on Vulkan — not recommended
 
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