docs: draw the example plots in the qc-core ledger style

[thomaspinder]

Summary

The example notebooks now draw their plots in the QuantClimate "Calibrated Ledger" style from qc-core, as the Impulso docs do. The docs site uses Impulso's oxblood accent scale.

 # every docs/examples/*.py
-from utils import use_mpl_style
 ...
-use_mpl_style()
+gpx.plotting.use_style()

The new public helper picks the style that it can load:

gpjax.plotting.use_style()
  import matplotlib            -> ImportError with an install hint if missing
  if qc_core imports
    qc_core.use_ledger_style()  -> returns "ledger"
  else
    style.use(gpjax/gpjax.mplstyle)  -> returns "gpjax"  (the old docs style, now packaged)

 gpjax/
+├── plotting.py          # use_style()
+└── gpjax.mplstyle       # moved from docs/examples/
 docs/
 ├── conf.py              # accent_color: red -> crimson
 ├── stylesheets/extra.css  # full crimson scale re-toned to oxblood (from impulso)
 └── examples/
-    ├── gpjax.mplstyle
     └── utils.py         # use_mpl_style removed
 pyproject.toml           # docs extra: qc-core>=0.0.4; qc-core exempt from exclude-newer

Notes:

• qc-core>=0.0.4 is needed: 0.0.1 and 0.0.2 set scatter.edgecolors to a bare hex, which makes every ax.scatter() raise (fixed in QuantClimate/core#2), and 0.0.4 adds the bundled Noto Sans font and a brighter second cycle colour (QuantClimate/core#3).
• qc-core is exempt from the 7-day exclude-newer cooldown (exclude-newer-package = { qc-core = false }), because it is our own package, published by trusted publishing. All other packages keep the cooldown.
• A hidden remove-cell in each notebook hides matplotlib's font-fallback log messages (the build host has no Public Sans/Spectral), as Impulso does.
• Dark-mode accent changes from #d07b76 to the ledger's #cf6f60.

Evidence

• Before: with qc-core 0.0.1, poe docs-ci failed with 12 warnings. 4 notebooks (classification, constructing_new_kernels, graph_kernels, spatial_linear_gp) raised ValueError: 'faf9f7' is not a valid color value in ax.scatter(), and 16 pages showed findfont messages.
  After: with qc-core 0.0.4 from PyPI, poe docs-ci passes with 0 warnings, and 0 pages show findfont output.
• tests/test_plotting.py:

  use_style() with qc_core installed   -> "ledger", figure.facecolor == #faf9f7
  use_style() with qc_core blocked     -> "gpjax", prop_cycle == packaged style
  use_style() with matplotlib blocked  -> ImportError("needs Matplotlib")
  packaged style file exists in the wheel
  

• poe test: 3233 passed, 1 skipped. poe lint and poe docstrings pass. The built wheel contains gpjax/plotting.py and gpjax/gpjax.mplstyle.

Merge Danger

Door: two-way

The docs and CSS changes revert cleanly. gpjax.plotting is a new public module: it is additive, but after a release, removing it would be a breaking change.

Blast Radius: docs

Every example figure changes appearance: new colours, fonts, and the paper background. The 5-colour ledger cycle replaces the 8-colour GPJax cycle (no notebook uses more than 5). Scatter markers change from x to circles, and the default colormap changes from inferno to Matplotlib's default (plots that set cmap explicitly keep it). Site links, hovers, and focus rings change to oxblood tints in both colour modes. Library users are not affected unless they call use_style(); matplotlib and qc-core stay optional.

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[github-actions]

📖 Docs preview: https://pr-803--endearing-crepe-c2d5fe.netlify.app

Smoke render — the expensive notebooks run with reduced budgets, so
figures are not publication fidelity. /render-mode.txt says smoke.