feat(geom_rect): add a hatch aesthetic

[floringogianu]

Fixes #238

from plotnine import *
from plotnine.data import mpg

(
    ggplot(mpg, aes("class", fill="drv", hatch="drv"))
    + geom_bar(position="dodge", color="black")
    + scale_hatch_manual(values=["//", "xx", ".."])
    + labs(x="vehicle class", y="count", fill="drive", hatch="drive")
)

Adds a hatch aesthetic to geom_rect and everything built on it (geom_bar, geom_col, geom_histogram, geom_tile, geom_bin_2d), settable as a literal (geom_col(hatch="//")) or mapped through a discrete scale (geom_col(aes(hatch="grp"))).

A matplotlib Collection takes one hatch pattern for all of its paths, so each distinct pattern gets an overlay PolyCollection with no fill and no border, drawn over the parent. The strokes take the color aesthetic, matplotlib's convention for a hatch, and fall back to rcParams["patch.edgecolor"] when color is unset. The overlay's linewidth is zero so the parent keeps sole ownership of the border and its linetype.

geom_polygon.draw_group carries the hatch through as well, since geom_rect delegates to it under a non-linear coord.

Legend keys repeat the pattern by round(HATCH_KEY_SIZE / key_size), so a small key stays legible while a large one converges on the density the panel draws.

Scale family modelled on scale_linetype:

• scale_hatch (discrete) / scale_hatch_discrete
• scale_hatch_identity
• scale_hatch_manual (custom per-level patterns)
• scale_hatch_continuous raises PlotnineError, mirroring scale_linetype_continuous

labs(hatch=...) names the legend.

[codecov]

Codecov Report

❌ Patch coverage is 92.50000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.22%. Comparing base (1f5ee9f) to head (4d20623).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
plotnine/geoms/geom_polygon.py	88.23%	2 Missing and 2 partials ⚠️
plotnine/scales/scale_hatch.py	93.10%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1122      +/-   ##
==========================================
+ Coverage   88.20%   88.22%   +0.02%     
==========================================
  Files         222      223       +1     
  Lines       16252    16331      +79     
  Branches     2080     2087       +7     
==========================================
+ Hits        14335    14408      +73     
- Misses       1315     1319       +4     
- Partials      602      604       +2     

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[has2k1]

@floringogianu thank you for taking this on. When I get to reviewing the PR, it will be faster for me to make the changes rather do a back and forth review.

I will get this in for the upcoming 0.16.0 release.

[floringogianu]

Sure, sounds good! I can probably take a look at the falling checks early next week.