pixtreme 1.8.0

pixtreme 1.8.0 allows separately installed font packages to contribute subpackages beneath pixtreme.fonts.
px.io.to_v210 now writes zero to sample slots with no corresponding pixel in the last six-pixel group of a row;
encoded bytes change for widths not divisible by six, while pixel data and px.io.from_v210 results stay the same.
px.metrics.ssim and px.metrics.ssim_map have about 34% lower median runtimes in the measured FHD case, and EXR
decoding has internal speed improvements. Public operation names, arguments, accepted names, and return shapes remain
compatible.
The published performance report (docs/performance.md) and README performance table still show the earlier
210-case baseline measured at commit 260fe6e; the SSIM improvements below were measured separately and those
tables have not been regenerated.

Added

• pixtreme.fonts is now a package, allowing a separately installed font package to contribute a
  pixtreme/fonts/<name>/ subtree and import it as pixtreme.fonts.<name>. The existing
  px.fonts.available() and px.fonts.font_path(name) operations and entry-point font discovery retain
  their behavior.

Changed

• px.metrics.ssim and px.metrics.ssim_map use a separable window calculation with compensated accumulation.
  On an NVIDIA RTX A6000 with FHD fp32 RGB inputs and data_range=1.0, the medians across three targeted runs
  fell from 2.014 to 1.328 ms and 2.025 to 1.342 ms, respectively (about 34%). Results remain within the
  existing 4e-5 tolerance against the independent float64 oracle.
• EXR decoding avoids repeated construction of channel rows and combines chunk replacements, improving its
  internal read path without changing the public API.

Fixed

• px.io.to_v210 now zeros unused Y, Cb, and Cr sample slots in the last six-pixel group of rows whose widths
  are not divisible by six. Encoded bytes for these widths differ from 1.7.0; samples that represent pixels,
  row padding, and px.io.from_v210 results are unchanged.