Unsigned BC6h sampleLower is negative

Hello 👋

the example DFD for unsigned BC6h in table 48 (around [here](https://www.khronos.org/registry/DataFormat/specs/1.3/dataformat.1.3.html#bc6h_channel), not sure how to link to the actual table) has a `sampleLower` of `0xBF800000U = -1.0f` although it's an unsigned format, and the signed flag is not set in the channel qualifiers.

Is this an error in the table or an intentional choice? dfdutils doesn't make this exception and [chooses `sampleLower = 0.0f`](https://github.com/KhronosGroup/dfdutils/blob/659a739bf60bdcd730b2159d658fb22e805854c2/createdfd.c#L571-L578) for `_UFLOAT` Vulkan types, contradicting the example.

Start with table 48 in the Data Format specification, around the BC6h channel section, and compare its unsigned sampleLower value with createdfd.c lines 571-578 in dfdutils. Determine whether the table value is erroneous or intentional, then document the decision and make the specification and example consistent.

Written by the indexing model from the issue text.