test(cpu): rename vmsum3fp_… to vmaddfp_lane_fma per reviewer nit

P8 review feedback (non-blocking): the test fn name said vmsum3fp but
the encoding/body actually tests vmaddfp. Rename + clarify comment;
no behavior change.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/xenia-cpu/src/interpreter.rs

@@ -7354,11 +7354,10 @@ mod tests {
         assert_eq!(r[15], 0xDD);
     }
 
-    // PPCBUG-490 VMX multiply-sum.
+    // PPCBUG-490 VMX multiply-add (vmaddfp; vmsum* covered indirectly by P5).
     #[test]
-    fn vmsum3fp_horizontal_3lane_sum() {
-        // vmsum3fp128 (already tested in P5 indirectly); here test scalar vmsum4ubm.
-        // Skip scalar VMX multiply-sum byte forms (large encoding); test vmaddfp.
+    fn vmaddfp_lane_fma() {
+        // Per-lane fused multiply-add: vD[i] = vA[i] * vC[i] + vB[i].
         let mut ctx = PpcContext::new();
         let mut mem = TestMem::new();
         ctx.vr[1] = xenia_types::Vec128::from_f32x4_array([2.0, 3.0, 4.0, 5.0]);