assert half assumptions

add static_assert to validate the assumptions of half:

/// * sizeof (float) == 4
/// * sizeof (unsigned int) == sizeof (float)
/// * alignof (unsigned int) == alignof (float)
/// * sizeof (unsigned short) == 2

And replace the int type with uintN_t

No file or test is named. Locate the implementation and uses of half, inspect how its assumptions and integer types are currently defined, and verify the listed static assertions and uintN_t replacements against the existing test suite or build checks.

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