[BCSharp]

System.Half had no Python-level support beyond what .NET exposes on its own: instances could only be created with clr.Convert, and several common Python operations silently produced wrong results. This PR started with adding a proper constructor to Half: in C#, it's easy to write (Half)0.5, but in IronPython, one needed the verbose clr.Convert(0.5, Half). There is no 16-bit sized float in CPython, but Half is basically the same as NumPy.float16, so I took that as the guide to add more functionality to Half.

The goal is to make Half behave correctly as a Python object, not to turn it into a Python float or even System.Single. Mixed-type arithmetic and comparisons (Half(1) + 1, Half(1) < 2.0) stay unsupported, as they are in C#, which has no implicit conversions between Half and other numeric types (though surprisingly, Byte gets automatically widened to Half in C#; not in IronPython). Same-type operators on Half are supported by the generic math in .NET.

Changes

	Before	After
Constructor	System.Half(1.5) → TypeError: CreateInstance() takes no arguments	Accepts the same arguments as System.Single(...): numbers, strings, objects with __float__
copy / pickle	Silently reconstructed Half(0)	Round-trips (via __getnewargs__, as for Single)
format(clr.Convert(1.5, Half), '.2f')	'22f' (spec passed to Half.ToString as a .NET custom format)	'1.50': formatted like a float with the same exact value; format(h, '') equals str(h)
str / repr	<System.Half object at 0x… [0.1]>; str was culture-dependent and lossy (65500 (as MaxValue), NaN, ∞)	Shortest round-trip string, culture-independent (6.55e+04 nan, inf)
Math.Abs(clr.Convert(1.5, Half))	1 (bound to Abs(Int32) via __int__); inf → 2147483647, nan → 0	1.5 (binds to the lossless Abs(Double))
real, imag, conjugate()	AttributeError	Implemented as for Single, typed as Half

[slozier]

Looks good to me. Just the small suggestion about __bool__ (unless it was intentionally omitted).

[slozier]

public static bool __bool__(Half self) => !self.IsZero;

might be worth it...