bug: make math.clamp invalid bounds deterministic

[vycdev2]

Summary

math.clamp delegates directly to Rust f64::clamp. Reversed bounds panic the interpreter instead of returning a controlled Jett runtime error; NaN bounds have the same host-panic behavior.

Reproduction

function main() returns nothing:
    float64 value = math.clamp(1.0, 2.0, 1.0)
    println(value)

cargo run -q -p jett_cli -- run <fixture> exits with Rust panic code 101 (min > max, or either was NaN) on current main.

Scope

• validate floating-point lower/upper bounds before invoking the host clamp implementation;
• return deterministic Jett runtime errors for reversed bounds and NaN bounds;
• add runtime-failure regressions proving invalid bounds no longer unwind the interpreter;
• preserve valid floating-point clamping behavior.

The public typechecker signature is math.clamp(float64, float64, float64) -> float64; the interpreter also contains an unreachable integer arm, which is outside this narrow fix. This is intentionally separate from #63, which excludes math.clamp pending an explicit invalid-bound/NaN contract.

Acceptance criteria

• Reversed floating-point bounds produce a controlled runtime error.
• NaN floating-point bounds produce a controlled runtime error.
• Existing valid math.clamp cases continue to pass.
• cargo fmt --check, cargo build, and cargo test -q pass.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.