pixtreme 1.6.0

pixtreme 1.6.0 adds two public capabilities. The first is the px.io.from_p216 / px.io.to_p216 pair, the ninth
named-format pair, which exchanges 16-bit 4:2:2 semi-planar frames (a uint16 Y plane followed by a uint16
interleaved Cb Cr plane) as one C-contiguous device array under the existing 4:2:2 planar rules; the io module grows
to 29 operations and the public API to 100. The second is the keyword-only preserve_shaper option of
px.io.read_lut and px.io.decode_lut, which evaluates a nonidentity .3dl shaper strictly in one GPU kernel
instead of the default same-edge bake. Both are independent additions: existing token spellings, aliases, signatures,
and float32 results are unchanged, and there are no breaking changes to the public API. The published performance
baseline (docs_site/performance.md and the README table) is regenerated from a single same-run measurement of the
current implementation on an NVIDIA RTX A6000 (WSL2, CUDA runtime 12.9, CuPy 14.1.1) and now covers the two P216
cases.

Added

• Added px.io.from_p216 and px.io.to_p216, the ninth named-format pair. P216 is 16-bit 4:2:2 semi-planar
  (a uint16 Y plane followed by a uint16 interleaved Cb Cr plane) exchanged as a C-contiguous 1D device array.
  The effective code depth is fixed at 16 bits with no bit_depth argument; range, rounding, clipping, the
  even-width constraint, horizontally co-sited chroma at full vertical resolution, and the interpolation tokens
  follow the existing 4:2:2 planar rules. The io module now exposes 29 operations and the public API 100.
• Added the keyword-only preserve_shaper option to px.io.read_lut and px.io.decode_lut. For nonidentity
  .3dl spacing, opting in retains a shared shaper on px.core.Lut and evaluates its linear stage with the existing
  trilinear or tetrahedral cube stage in one GPU kernel. The default remains the existing same-edge baked
  approximation. Writing a shaped LUT as Cube performs the same-edge bake on the GPU; the independent shaper and
  strict between-node evaluation are not recoverable from that output.