Fix incorrect CPU float16 implementation

[eliasachermann]

Problem

The dace::half fallback in dace/runtime/include/dace/types.h is broken in two independent ways:

1. No default constructor.
   Generated code that declares an uninitialized dace::float16 fails to compile:

   error: no matching function for call to 'dace::half::half()'
   

2. Incorrect conversions.
   The float↔half conversion is wrong for zero, subnormals, Inf/NaN, and does no rounding.

Fix

• Replace both conversions with IEEE-754 round-to-nearest-even routines, correctly handling zero, subnormals, overflow→Inf and Inf/NaN.
• Add a default constructor.

Tests

Adds tests/half_cpu_test.py, to test the conversion against NumPy as the reference.

[ThrudPrimrose]

We need subnormal, inf and NaN in roundtrip tests to ensure we handle them correctly.

[ThrudPrimrose]

Can you add a subnormal number to your tests?

[eliasachermann]

Added inf and nan tests

[acalotoiu]

LGTM