Fix HexTile centerline cell selection

Evaluate the third cube coordinate symmetrically to retain the intended centerline tie. Keep the existing pixel oracle and tolerance.

Scope the Mem fixture mutex unwrap allowance so the original strict CI can validate this base.

Co-authored-by: Codex <codex@openai.com>

crates/gpu/src/shaders/fx_pixel2.wgsl

@@ -647,7 +647,9 @@ fn fp2_hextile(@builtin(global_invocation_id) gid: vec3<u32>) {
     // stylize3::hex_cell (cube rounding).
     let q = (0.5773502691896257 * u - v / 3.0) / r;
     let rr = (2.0 / 3.0 * v) / r;
-    let ccy = -q - rr;
+    // Evaluate the third cube coordinate symmetrically with q so the centerline
+    // tie is not broken by subtracting separately rounded coordinates.
+    let ccy = (-0.5773502691896257 * u - v / 3.0) / r;
     var rx = round_away(q);
     let ry = round_away(ccy);
     var rz = round_away(rr);

crates/script/src/tests_services.rs

@@ -11,6 +11,8 @@ use crate::run_code;
 #[derive(Default)]
 struct Mem(Mutex<HashMap<String, Vec<u8>>>);
 
+// A poisoned fixture mutex should fail the test.
+#[allow(clippy::unwrap_used)]
 impl Services for Mem {
     fn read_file(&self, path: &str) -> std::io::Result<Vec<u8>> {
         self.0.lock().unwrap().get(path).cloned().ok_or_else(|| std::io::Error::new(std::io::ErrorKind::NotFound, path.to_string()))