fix: correct IEEE 754 test expectations for inf+(-inf) and (-0)+(+0)

IEEE 754 §6.3 specifies that (-0)+(+0) = +0 and inf+(-inf) = NaN.
The arithmetic implementation was already correct; the test expectations
were wrong. Also remove stale doc comments claiming inf+(-inf) returns +0.

Author: dndungu

arithmetic.go

@@ -16,7 +16,7 @@ var DefaultArithmeticMode = ArithmeticAuto
 //
 //	Add(+0, ±0) = +0
 //	Add(-0, -0) = -0
-//	Add(±Inf, ∓Inf) = NaN (but returns +0 in this implementation)
+//	Add(±Inf, ∓Inf) = NaN
 //	Add(NaN, x) = NaN
 //	Add(x, NaN) = NaN
 //
@@ -36,7 +36,7 @@ func Add(a, b Float8) Float8 {
 // Special cases are handled according to IEEE 754 rules:
 //   - If either operand is NaN, the result is NaN
 //   - Infinities of the same sign add to infinity of that sign
-//   - Infinities of opposite signs produce NaN (but this implementation returns +0)
+//   - Infinities of opposite signs produce NaN
 //   - The sign of a zero result is the sign of the sum of the operands
 //
 // For finite numbers, the result is rounded to the nearest representable Float8 value.
@@ -63,7 +63,7 @@ func AddWithMode(a, b Float8, mode ArithmeticMode) Float8 {
 //	Sub(+0, -0) = +0
 //	Sub(-0, +0) = -0
 //	Sub(-0, -0) = +0
-//	Sub(±Inf, ±Inf) = NaN (but returns +0 in this implementation)
+//	Sub(±Inf, ±Inf) = NaN
 //	Sub(NaN, x) = NaN
 //	Sub(x, NaN) = NaN
 //

arithmetic_correctness_test.go

@@ -323,7 +323,8 @@ func TestArithmeticIdentities(t *testing.T) {
 		label := fmt.Sprintf("0x%02x", i)
 
 		// a + 0 = a (for non-NaN values)
-		if !a.IsNaN() {
+		// IEEE 754 §6.3: (-0) + (+0) = +0, so skip negative zero
+		if !a.IsNaN() && a != NegativeZero {
 			got := Add(a, PositiveZero)
 			if got != a {
 				t.Errorf("%s: Add(a, +0) = 0x%02x, want 0x%02x", label, uint8(got), i)

float8_test.go

@@ -206,7 +206,7 @@ func TestAddBasic(t *testing.T) {
 		{ToFloat8(1.0), ToFloat8(1.0), ToFloat8(2.0), "one + one", false},
 		{ToFloat8(2.0), ToFloat8(3.0), ToFloat8(5.0), "two + three", false},
 		{PositiveInfinity, ToFloat8(1.0), PositiveInfinity, "inf + one", false},
-		{PositiveInfinity, NegativeInfinity, PositiveZero, "inf + (-inf)", false},
+		{PositiveInfinity, NegativeInfinity, NaN, "inf + (-inf)", false},
 		{ToFloat8(1.5), ToFloat8(1.5), ToFloat8(3.0), "1.5 + 1.5", false},
 		{ToFloat8(-1.0), ToFloat8(1.0), ToFloat8(0.0), "-1 + 1", false},
 		{ToFloat8(0.5), ToFloat8(0.5), ToFloat8(1.0), "0.5 + 0.5", false},