test: add gemv per-dispatch overhead micro-bench

Tests/FFAITests/Benchmark/DispatchOverheadBench.swift

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+// Copyright 2026 Eric Kryski (@ekryski) and Tom Turney (@TheTom)
+// SPDX-License-Identifier: Apache-2.0
+//
+// Swift -> Metal per-dispatch overhead, the apples-to-apples counterpart to the
+// Rust `dispatch_bench`. Measures one gemv two ways:
+//   - per-commit: one command buffer per op, commit + wait (matches the Rust
+//     Device.dispatch model exactly).
+//   - batched: many ops encoded into ONE command buffer, one commit (what the
+//     Swift `Ops.gemv(on: cmd)` API enables, and where the real headroom is).
+// The per-commit number isolates the Metal commit()+wait floor; if it lands
+// near the Rust number, the host language is not the variable.
+
+import Foundation
+import Metal
+import Testing
+
+@testable import FFAI
+
+@Suite("Dispatch overhead bench")
+struct DispatchOverheadBench {
+    @Test("gemv per-dispatch overhead: per-commit vs batched")
+    func gemvDispatchOverhead() {
+        let m = 2048
+        let w = Tensor.empty(shape: [m, m], dtype: .f32)
+        let x = Tensor.empty(shape: [m], dtype: .f32)
+
+        // warmup (PSO build + first encode)
+        for _ in 0..<20 {
+            let cmd = Device.shared.makeCommandBuffer()
+            _ = Ops.gemv(weight: w, input: x, on: cmd)
+            cmd.commit()
+            cmd.waitUntilCompleted()
+        }
+
+        let iters = 3000
+
+        // per-commit: one command buffer per dispatch (matches Rust)
+        var t0 = Date()
+        for _ in 0..<iters {
+            let cmd = Device.shared.makeCommandBuffer()
+            _ = Ops.gemv(weight: w, input: x, on: cmd)
+            cmd.commit()
+            cmd.waitUntilCompleted()
+        }
+        let perCommitUs = Date().timeIntervalSince(t0) * 1e6 / Double(iters)
+
+        // batched: encode N into one command buffer, single commit
+        let batch = 500
+        t0 = Date()
+        let cmd = Device.shared.makeCommandBuffer()
+        for _ in 0..<batch {
+            _ = Ops.gemv(weight: w, input: x, on: cmd)
+        }
+        cmd.commit()
+        cmd.waitUntilCompleted()
+        let batchedUs = Date().timeIntervalSince(t0) * 1e6 / Double(batch)
+
+        print("Swift -> Metal gemv (2048x2048):")
+        print(String(format: "  per-commit (1 cmd buffer/op): %.1f us/dispatch", perCommitUs))
+        print(String(format: "  batched (%d ops / 1 cmd buffer): %.1f us/op", batch, batchedUs))
+        print("  (Rust per-commit, resident weights, was 178 us/dispatch)")
+    }
+}