trunc_sat instructions lack proper saturation (NaN/out-of-range -> UB)

[saza-ku]

trunc_sat instructions must saturate on NaN and out-of-range inputs, but some current implementations appear to be plain trunc (e.g., I32TruncSatF64S uses build_float_to_signed_int without clamping). This can lead to undefined behavior for NaN/out-of-range values.

Affected area:

• mod.rs

Expected behavior:

• NaN → 0
• Out-of-range → clamp to min/max

[saza-ku]

related?: #15