use math.powi/pow for power operator instead of bitwise xor

[ylluminate]

Python's ** operator was mapped to V's ^ which is bitwise XOR, not exponentiation. 2 ** 10 produced 2 ^ 10 = 8 instead of 1024.

Uses math.powi() for integer operands (preserves int type) and math.pow() for float operands or negative exponents (Python auto-promotes 2**-1 to 0.5). Also handles **= augmented assignment by expanding to assignment form since V has no **= operator.

Changes

• visit_binop() in transpiler.v: Pow handler with type-aware dispatch to math.powi (int) or math.pow (float/negative exponent)
• visit_aug_assign() in transpiler.v: Pow special-case expands a **= b to a = math.powi(a, b) or a = math.pow(a, b)
• Negative exponent detection: UnaryOp(USub, ...) forces float path
• op_to_symbol() in types.v: Pow maps to ** (not a V operator)
• Test cases added for **= augmented assignment and negative exponents
• 3 test expected outputs updated (numeric_ops, math_func)

All 110 tests pass.

Known limitations (follow-up)

• math.powi returns i64, may need cast in typed contexts
• Variable negative exponents (x = -1; 2 ** x) not detected at transpile time

[ylluminate]

Closing for revision — will resubmit after review.