emitCmpNe lowered float `!=` to `LLVMRealONE` (ordered not-equal), which
is false when either operand is NaN. That made `nan != nan` false in
native code — breaking the canonical `x != x` NaN test, making `!=`
non-complementary with `==` for NaN, and disagreeing with the interpreter.
Change the float predicate to `LLVMRealUNE` (unordered not-equal): true
if either operand is NaN OR they are unequal. For all non-NaN operands
`UNE` ≡ `ONE`, so only NaN-involving comparisons change (toward correct).
The integer predicate (`LLVMIntNE`) and `emitCmpEq` (`OEQ`) are unchanged,
so `nan == nan` stays false and `!=` is now the exact complement of `==`.
- Regression: examples/0150-types-float-ne-unordered-nan.sx (fails before,
passes after; also pins #run/comptime == runtime agreement).
- specs.md: documents float comparison / NaN semantics (Operators).
- Resolves issue 0091 (issues/0091-float-ne-ordered-nan.md).
