fix(bench): give query-benchmark.ts's diffImpact metric a warmup too

scripts/query-benchmark.ts

@@ -109,13 +109,18 @@ console.log = (...args) => console.error(...args);
 
 const RUNS = 5;
 
-// First 2-3 native fnDeps calls per process pay a cold-start cost (rusqlite
-// statement-cache warmup, OS page cache for the DB file, NAPI-side static
-// init from tree-sitter's transitive crates linked into the .node binary).
-// On Linux x86_64 CI, that pulled median(5) into cold-start territory once
-// tree-sitter 0.25 grew the binary's init footprint (#1076), even though
-// steady-state per-call latency is unchanged. Discard the first WARMUP_RUNS
-// before timing so the metric reflects warm-call latency, not cold-start.
+// The first 2-3 native calls into a given query path per process pay a
+// cold-start cost (rusqlite statement-cache warmup, OS page cache for the DB
+// file, NAPI-side static init from tree-sitter's transitive crates linked
+// into the .node binary). On Linux x86_64 CI, that pulled median(5) into
+// cold-start territory once tree-sitter 0.25 grew the binary's init footprint
+// (#1076), even though steady-state per-call latency is unchanged. Discard
+// the first WARMUP_RUNS before timing so the metric reflects warm-call
+// latency, not cold-start.
+//
+// This applies per query path, not just once per process: each of fnDeps,
+// fnImpact and diffImpact prepares its own statements and touches its own DB
+// pages, so warming one does not warm the next (#2590).
 const WARMUP_RUNS = 3;
 
 async function benchDepths(fn, name, depths) {
@@ -166,6 +171,17 @@ async function benchDiffImpact(targets: HubTargets) {
 		fs.writeFileSync(hubFile, original + '\n// benchmark-probe\n');
 		execFileSync('git', ['add', hubFile], { cwd: root, stdio: 'pipe' });
 
+		// Warm the diffImpact path before timing, exactly as benchDepths does for
+		// fnDeps/fnImpact. diffImpactData is a distinct query path from those --
+		// its own prepared statements, its own DB pages, and a `git` subprocess to
+		// read the staged diff -- so the warmup those calls already paid does not
+		// carry over. Without this, diffImpact was the only query metric measured
+		// cold: the same defect #2584 fixed for benchmark.ts's Full build, and
+		// that #2436 traced part of the gate's non-determinism to (#2590).
+		for (let i = 0; i < WARMUP_RUNS; i++) {
+			diffImpactData(dbPath, { staged: true, depth: 3, noTests: true });
+		}
+
 		let lastResult = null;
 		const latencyMs = round1(
 			await timeMedian(() => {