benches: add SMARM_BENCH_SETS (default 5) for stable medians

Each run_n call now collects ITERS × SETS raw samples across all sets
(one warmup before the first set, discarded), then takes a single global
median. Previously only ITERS=15 samples were taken per bench invocation,
which produced noisy results in sweep.py regress.

SMARM_BENCH_SETS is read from the environment; the default of 5 gives
75 samples per bench row (5×15), matching the stability of the multi-run
bash approach without requiring multiple cargo bench invocations.

Also moves scripts/bench_rq.sh into benches/ alongside sweep.py.
The cd "$(dirname "$0")/.." path logic is unchanged.

benches/general.rs

@@ -29,6 +29,13 @@ fn available_threads() -> usize {
     std::thread::available_parallelism().map(|n| n.get()).unwrap_or(1)
 }
 
+fn env_sets() -> u32 {
+    std::env::var("SMARM_BENCH_SETS")
+        .ok()
+        .and_then(|v| v.parse().ok())
+        .unwrap_or(5)
+}
+
 fn print_header(title: &str) {
     println!("\n{}", "=".repeat(80));
     println!("  {title}");
@@ -41,14 +48,17 @@ fn print_header(title: &str) {
 }
 
 fn run_n<F: FnMut() -> (u64, u128)>(name: &str, n: u32, mut f: F) {
-    let mut times = Vec::new();
+    let sets = env_sets();
+    let mut times = Vec::with_capacity((n * sets) as usize);
     let mut last = 0u64;
-    // One warmup iteration, discarded.
+    // One warmup before all sets, discarded.
     let _ = f();
-    for _ in 0..n {
-        let (v, t) = f();
-        times.push(t);
-        last = v;
+    for _ in 0..sets {
+        for _ in 0..n {
+            let (v, t) = f();
+            times.push(t);
+            last = v;
+        }
     }
     times.sort_unstable();
     let median = times[times.len() / 2];
@@ -406,7 +416,8 @@ fn main() {
     let n = available_threads();
     println!("smarm general benchmarks");
     println!("available parallelism: {n} threads");
-    println!("ITERS={ITERS} (+1 warmup, discarded)");
+    let sets = env_sets();
+    println!("ITERS={ITERS}×{sets} sets = {} samples (+1 warmup, discarded)", ITERS * sets);
     println!(
         "CHAIN_DEPTH={CHAIN_DEPTH}, YIELD_TASKS={YIELD_TASKS}×{YIELD_ROUNDS}, \
          PRIME_N={PRIME_N}/{PRIME_WORKERS} workers, PP_ROUNDS={PP_ROUNDS}"