fix(backend): float != must be UNORDERED so nan != nan is true [F0.9]

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).

examples/expected/0150-types-float-ne-unordered-nan.exit

@@ -0,0 +1 @@
+0

examples/expected/0150-types-float-ne-unordered-nan.stderr

@@ -0,0 +1 @@
+

examples/expected/0150-types-float-ne-unordered-nan.stdout

@@ -0,0 +1,8 @@
+nan != nan: true
+nan == nan: false
+nan != 1.0: true
+nan == 1.0: false
+1.0 != 2.0: true
+1.0 != 1.0: false
+2.0 != 2.0: false
+comptime nan != nan: true