PERF-2 P1.b: the polyline index prunes from the start with an upper bound from each chunk's first vertex (never the answer; the strict-minimum scan rule is unchanged), so a query near the polygon's far end no longer scans every chunk; pin unchanged, 0 mismatches

crates/specialized/rtx-cfd/src/mesh/patch_gen.rs

@@ -330,10 +330,21 @@ impl<'a> PolylineIndex<'a> {
         let pts = self.pts;
         let n = pts.len();
         let (mut best, mut best_d) = (pts[0], f64::INFINITY);
+        // An UPPER bound on the minimum from each chunk's first vertex (a
+        // point of the polyline, so no segment can beat it by more than the
+        // rounding margin): chunks farther than it are pruned from the
+        // start, and the bound never becomes the answer — the scan below
+        // keeps the brute-force rule (first segment at the strict minimum).
+        let mut prune_d = f64::INFINITY;
+        for c in 0..self.boxes.len() {
+            let a = pts[c * self.chunk];
+            let d = (a[0] - q[0]).powi(2) + (a[1] - q[1]).powi(2);
+            prune_d = prune_d.min(d);
+        }
         for (c, b) in self.boxes.iter().enumerate() {
             let dx = (b[0] - q[0]).max(0.0).max(q[0] - b[1]);
             let dy = (b[2] - q[1]).max(0.0).max(q[1] - b[3]);
-            if dx * dx + dy * dy > best_d * (1.0 + 1e-12) {
+            if dx * dx + dy * dy > best_d.min(prune_d) * (1.0 + 1e-12) {
                 continue;
             }
             let start = c * self.chunk;