Add plotstyle: reusable KIT-branded matplotlib styling toolkit

Introduces a new plotstyle package providing consistent, presentation/thesis-ready
matplotlib figures: a KIT corporate-design color palette (categorical, sequential,
diverging, status), a theme applied via use() (KIT-black bottom spine only, left/bottom
ticks, horizontal gridlines, left-aligned titles, LaTeX text in Latin Modern Sans),
new_figure() with size presets and figure-level title/params subtitles, a
same-size colorbar() helper, style_legend() and panel_label() building blocks, and
savefig(). Ships as an optional "plotting" extra so core gallery installs stay
lightweight, with a full test suite and a runnable example notebook.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

plotstyle/colors.py

@@ -0,0 +1,102 @@
+"""KIT (Karlsruhe Institute of Technology) corporate design color palette.
+
+Hex values for KIT green, KIT blue, black 70%, and the corporate accent
+colors are taken verbatim from the KIT corporate design guide
+(https://kit-cd.km.kit.edu/english/341.php) — do not hand-edit them without
+checking that page.
+
+The categorical *order* below is not arbitrary: it was chosen by running
+every hue through the CVD-safety/contrast checks described in the `dataviz`
+skill (fixed hue order, OKLab CVD separation under simulated color-vision
+deficiency, a normal-vision separation floor, contrast vs. a white surface)
+and keeping the ordering that clears the adjacent-pair checks. KIT yellow
+(#FCE500) is the one hue that cannot pass on its own (too light on a white
+surface, ~1.3:1 contrast) — that is a property of the hex value itself, not
+the ordering, so it is placed last and should always be paired with a
+visible direct label rather than relied on as a fill alone.
+"""
+
+from __future__ import annotations
+
+from matplotlib.colors import LinearSegmentedColormap
+
+# Fixed-order categorical hues (KIT primary + accent colors). Order is the
+# CVD-safety mechanism: never reorder, and never cycle past the last slot
+# (fold extra series into "Other").
+CATEGORICAL = [
+    "#009682",  # 0 KIT green (primary)
+    "#DF9B1B",  # 1 orange
+    "#4664AA",  # 2 KIT blue (primary)
+    "#A78230",  # 3 brown
+    "#23A1E0",  # 4 cyan
+    "#A3107C",  # 5 purple
+    "#8CB63C",  # 6 pea green
+    "#A22223",  # 7 red
+    "#FCE500",  # 8 yellow — low contrast on white; always pair with a direct label
+]
+
+# Single-hue (KIT blue) sequential ramp, steps 100..700. Step 700 is the exact
+# brand hex (the high/saturated end); lighter steps are tints blended toward
+# white in sRGB — there is no darker-than-brand shade.
+SEQUENTIAL_STEPS = [
+    "#e3e8f2",  # 100
+    "#c9d2e6",  # 200
+    "#afbcda",  # 300
+    "#95a6ce",  # 400
+    "#7a90c2",  # 500
+    "#607ab6",  # 600
+    "#4664AA",  # 700 (KIT blue)
+]
+
+# Diverging KIT blue <-> KIT red, neutral gray midpoint.
+DIVERGING = {
+    "low": "#4664AA",
+    "mid": "#f7f7f7",
+    "high": "#A22223",
+}
+
+# Fixed, reserved status scale — deliberately NOT re-themed to KIT colors:
+# status is a small fixed scale with reserved meaning that must stay visually
+# distinct from the categorical slots so it never impersonates a series.
+# Never put these in the categorical cycle; always pair with an icon/label.
+STATUS = {
+    "good": "#0ca30c",
+    "warning": "#fab219",
+    "serious": "#ec835a",
+    "critical": "#d03b3b",
+}
+
+# Chrome / ink roles, derived from KIT's specified "black 70%" (#404040,
+# used by KIT for headings and continuous text) on a white surface.
+INK = {
+    "surface": "#ffffff",
+    "primary": "#404040",  # KIT black 70% — headings, titles, continuous text
+    "secondary": "#6a6a6a",
+    "muted": "#969696",
+    "gridline": "#ececec",
+    "baseline": "#c2c2c2",
+}
+
+
+def get_color(index: int) -> str:
+    """Return the categorical color for series `index` (0-based).
+
+    Raises ValueError past the validated slots instead of silently wrapping
+    back to slot 0, which would collide two series on the same hue.
+    """
+    if not 0 <= index < len(CATEGORICAL):
+        raise ValueError(
+            f"get_color({index}) out of range: only {len(CATEGORICAL)} validated categorical "
+            "colors exist. Fold extra series into an 'Other' bucket or facet instead of cycling."
+        )
+    return CATEGORICAL[index]
+
+
+def sequential_cmap(name: str = "ps_sequential") -> LinearSegmentedColormap:
+    """Continuous KIT-blue sequential colormap for magnitude encoding."""
+    return LinearSegmentedColormap.from_list(name, SEQUENTIAL_STEPS)
+
+
+def diverging_cmap(name: str = "ps_diverging") -> LinearSegmentedColormap:
+    """Continuous KIT blue-gray-red diverging colormap for polarity encoding."""
+    return LinearSegmentedColormap.from_list(name, [DIVERGING["low"], DIVERGING["mid"], DIVERGING["high"]])