Add polar-aware rlabel and thetalabel support

Hi! I think having rlabel and thetalabel for polar plots is useful for many
cases — Taylor diagrams being one example. I've added this support along with
tests. Let me know what you think!

Summary

This PR adds polar-aware axis label support to PolarAxes.format() with new
thetalabel and rlabel arguments rendered via CurvedText.

Key Features

• Add thetalabel for labels that follow the outer polar arc.
• Add rlabel for labels that follow a radial spoke.
• Add thetalabelloc to control the center angle of the theta label.
• Add rlabelloc to control which side or spoke the radial label uses.
• Support explicit rlabelpos placement for radial labels.
• Support styling through thetalabel_kw and rlabel_kw.
• Keep label placement correct for:
  • full-circle plots
  • sector plots
  • wrapped theta intervals
  • transformed theta orientation (theta0, thetadir)
  • annular polar axes
• Refresh label geometry during draw and tight-bbox updates.

Implementation

• Add polar-label geometry and state handling in ultraplot/axes/polar.py
• Update CurvedText in ultraplot/text.py so transformed and styled curved labels
  behave correctly on polar axes
• Split pseudo label props in ultraplot/internals/labels.py so border and bbox
  styling can be applied safely to curved labels

Validation

Focused polar regression tests pass:

python -m pytest ultraplot/tests/test_projections.py -k "polar and not mpl_image_compare"

This also adds regression coverage for:

• transformed thetalabel outward placement
• annular polar label positioning with finite glyph positions

Examples

Examples of full-circle, sector, annular, and rotated-axis polar labels:

import ultraplot as uplt

fig, axs = uplt.subplots(
    proj='polar', ncols=2, nrows=3, share=0,
)

axs[0, 0].format(
    thetalabel='Full circle',
    rlabel='Radius',
    title='Full circle, default locs',
    labelsize=10,
)

axs[0, 1].format(
    thetalim=(0, 360),
    rlim=(5, 10),
    thetalabel='Label at 250°',
    thetalabelloc=250,
    rlabel='left label',
    rlabelloc='left',
    title='Full circle, custom locs',
    labelsize=10,
)

axs[1, 0].format(
    thetalim=(0, 90),
    rlim=(0, 1),
    thetalabel='Quarter arc',
    rlabel='r [0-1]',
    title='Section, default locs',
    labelsize=10,
)

axs[1, 1].format(
    thetalim=(0, 180),
    rlim=(0, 10),
    thetalabel='Label at 135°',
    thetalabelloc=135,
    rlabel='Left spoke',
    rlabelloc='left',
    title='Section, custom locs',
    labelsize=10,
)

axs[2, 0].format(
    thetalim=(50, 70),
    r0=0, rlim=(1, 5),
    thetalabel='Narrow section',
    thetalabel_kw={'color': 'steelblue', 'style': 'italic'},
    rlabel='labelpad=50',
    rlabel_kw={'labelpad': 50, 'weight': 'bold', 'size': 13},
    title='Section, labelpad',
    labelsize=10,
)

axs[2, 1].format(
    thetalim=(0, 180),
    rlim=(2, 10),
    theta0='N',
    thetadir=-1,
    thetalabel='Reversed theta direction',
    rlabel='Radius',
    rlabelloc='right',
    title="theta0='N', thetadir=-1",
)
axs.format(
    suptitle='Polar labels demo',
    titlepad=30,
)