fix(cpu): PPCBUG-221+227 round_to_i64 + PPCBUG-432 vrfin round-to-even

Phase 5 batch 1 (5a): round-to-int correctness.

PPCBUG-221+227 (coupled): round_to_i64 NearestEven tie-breaking used
`(diff - 0.5).abs() < f64::EPSILON` to detect half-integers, but for
|v| > 2^52 every f64 value is an exact integer (v.trunc() == v), giving
diff == 0. The buggy check fell through to v.round() (round-half-away-
from-zero), giving wrong results for large odd half-integers. Replaced
with a fractional-part-only check that's exact for |v| <= 2^52 and
degenerates to truncation above.

PPCBUG-432: vrfin/vrfin128 used Rust's `f32::round()` which is round-
half-away-from-zero. ISA requires round-to-nearest-even (banker's
rounding). Implemented inline.

PPCBUG-201 (FPSCR.RN for double arithmetic) deferred — requires
MXCSR-set/restore wrappers around 10+ FPU arms; will land in a focused
sub-batch after the remaining 5a-5f fixes.

Tests:
- round_to_i64_nearest_even_on_tie: extended with 0.5, 1.5, -0.5, -1.5.
- round_to_i64_non_tie_cases: 0.4/0.6 (non-tie sanity).
- round_to_i32_nearest_even_on_tie: PPCBUG-227 coverage.

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

crates/xenia-cpu/src/fpscr.rs

@@ -220,15 +220,22 @@ pub fn round_to_single(ctx: &PpcContext, v: f64) -> f64 {
 pub fn round_to_i64(ctx: &PpcContext, v: f64) -> i64 {
     match rounding_mode(ctx) {
         RoundingMode::NearestEven => {
-            // Round-half-to-even (banker's rounding).
-            let r = v.round();
-            // Rust's f64::round is round-half-away-from-zero. Correct ties to even:
-            let diff = (v - v.trunc()).abs();
-            if (diff - 0.5).abs() < f64::EPSILON {
-                let floor = v.floor();
-                if (floor as i64) & 1 == 0 { floor as i64 } else { v.ceil() as i64 }
+            // PPCBUG-221: round-half-to-even (banker's rounding). The previous
+            // tie-detection used `(diff - 0.5).abs() < f64::EPSILON` which
+            // breaks for |v| > 2^52 (where v.trunc() == v exactly, giving diff
+            // == 0). Use a fractional-part-only check that's exact for
+            // |v| <= 2^52 and degenerates correctly above.
+            let t = v.trunc();
+            let frac = v - t;
+            let fa = frac.abs();
+            if fa > 0.5 {
+                t as i64 + if v >= 0.0 { 1 } else { -1 }
+            } else if fa < 0.5 {
+                t as i64
             } else {
-                r as i64
+                // Exact 0.5 tie — round to even.
+                let fi = t as i64;
+                if fi & 1 == 0 { fi } else { fi + if v >= 0.0 { 1 } else { -1 } }
             }
         }
         RoundingMode::TowardZero => v.trunc() as i64,
@@ -355,11 +362,35 @@ mod tests {
     #[test]
     fn round_to_i64_nearest_even_on_tie() {
         let c = ctx();
+        assert_eq!(round_to_i64(&c, 0.5_f64), 0);
+        assert_eq!(round_to_i64(&c, 1.5_f64), 2);
         assert_eq!(round_to_i64(&c, 2.5_f64), 2);
         assert_eq!(round_to_i64(&c, 3.5_f64), 4);
+        assert_eq!(round_to_i64(&c, -0.5_f64), 0);
+        assert_eq!(round_to_i64(&c, -1.5_f64), -2);
         assert_eq!(round_to_i64(&c, -2.5_f64), -2);
     }
 
+    #[test]
+    fn round_to_i64_non_tie_cases() {
+        // PPCBUG-221 regression: non-tie fractions must round to nearest.
+        let c = ctx();
+        assert_eq!(round_to_i64(&c, 0.4_f64), 0);
+        assert_eq!(round_to_i64(&c, 0.6_f64), 1);
+        assert_eq!(round_to_i64(&c, -0.4_f64), 0);
+        assert_eq!(round_to_i64(&c, -0.6_f64), -1);
+    }
+
+    #[test]
+    fn round_to_i32_nearest_even_on_tie() {
+        // PPCBUG-227: round_to_i32 inherits round_to_i64's tie semantics.
+        let c = ctx();
+        assert_eq!(round_to_i32(&c, 0.5_f64), 0);
+        assert_eq!(round_to_i32(&c, 1.5_f64), 2);
+        assert_eq!(round_to_i32(&c, 2.5_f64), 2);
+        assert_eq!(round_to_i32(&c, -1.5_f64), -2);
+    }
+
     #[test]
     fn check_invalid_add_detects_inf_minus_inf() {
         let mut c = ctx();