[Icons] Add a badge icon treatment

Restore the filled-disc look the ready/not-ready icons had before the
one-glyph cutover, without bringing back palette-locked art.

IconStyle::Badge composes the existing sentinel SVG at paint time: a
gradient disc, a dark ring, and the glyph re-weighted and inked for
contrast against the tone. Line stays the default and is unchanged.

Only the two state icons opt in. The add/remove icons keep the line
treatment because their old art was a filled glyph with an outline and
no disc, so badging them would invent a look the UI never had. Disabled
badges fall back to the line glyph so an unavailable control still reads
as unavailable.

cockatrice/src/interface/theme_icon_badge.h

@@ -0,0 +1,83 @@
+/**
+ * @file theme_icon_badge.h
+ * @ingroup Interface
+ */
+
+#ifndef THEME_ICON_BADGE_H
+#define THEME_ICON_BADGE_H
+
+#include <QByteArray>
+#include <QColor>
+#include <QPixmap>
+#include <QSize>
+
+/**
+ * @brief Paints the badge treatment of a `currentColor` glyph.
+ *
+ * A tint alone can say "positive" or "negative", but a 2px line at 16px reads as
+ * a different thing from the filled badge art the UI used before the one-glyph
+ * cutover. The badge treatment keeps that shape: a filled disc, a dark ring, and
+ * a contrast glyph, all derived from the one tone the tint resolves to. So the
+ * theme still owns the color, and callers only choose between a line and a
+ * badge.
+ *
+ * The trade-off is fidelity against contrast. A badge is a small picture: it
+ * survives a busy toolbar and a tinted row better than a hairline, and it can
+ * carry a light or dark glyph independent of the surrounding text. In exchange
+ * it costs more pixels of attention per control, and its glyph color is decided
+ * from the disc it sits on rather than from the window, so it is deliberately
+ * not the palette's text color.
+ *
+ * Everything here works in device pixels. The icon engine renders at the
+ * device-pixel size it already computed and sets the pixmap's device pixel ratio
+ * afterwards, so badge geometry must not assume logical pixels.
+ */
+namespace ThemeIconBadge
+{
+/**
+ * @brief WCAG relative luminance, used to test a color against a background.
+ *
+ * Shared by the badge glyph choice and by IconTint::resolve()'s selected-row
+ * contrast test, so both use the same definition of "legible".
+ */
+double relativeLuminance(const QColor &color);
+
+/** @brief WCAG contrast ratio between two opaque colors, from 1.0 to 21.0. */
+double contrastRatio(const QColor &first, const QColor &second);
+
+/**
+ * @brief Glyph color for a badge disc.
+ *
+ * The more legible of white and the tone darkened to an outline-dark ink. White
+ * keeps the old light-scheme look; on a dark scheme the semantic tone is a light
+ * green or red, where white sits near 1.7:1 and disappears, so the dark ink
+ * wins instead. Both candidates derive from the theme, so no literal is needed.
+ */
+QColor glyphColor(const QColor &tone);
+
+/** @brief Size of the engraved glyph for a badge of @p pixelSize, in device pixels. */
+QSize glyphSize(const QSize &pixelSize);
+
+/**
+ * @brief Renders a sentinel-carrying SVG as a badge.
+ *
+ * The disc fills the square canvas minus the ring, the ring darkens the tone,
+ * and the sentinel is resolved to the contrast glyph. The SVG is scaled to
+ * nearly the whole disc, because the sentinel paths are padded inside their
+ * viewBox and a smaller box loses the stroke at 16px.
+ *
+ * This treatment reproduces the pre-#7595 ready/not-ready art, which was a
+ * filled disc. It is not a generic replacement for every semantic glyph: the
+ * old add/remove icons were filled glyphs with an outline and no disc, so
+ * badging them would invent a look the UI never had.
+ *
+ * @param svg Sentinel-carrying SVG source; `currentColor` is resolved internally.
+ * @param tone Disc color, already resolved for the icon mode.
+ * @param pixelSize Canvas size in device pixels; a non-square canvas centers
+ *                  the badge and leaves the remaining area transparent.
+ * @return The composited badge, or a null pixmap for empty or invalid input.
+ */
+QPixmap renderBadge(const QByteArray &svg, const QColor &tone, const QSize &pixelSize);
+} // namespace ThemeIconBadge
+
+#endif // THEME_ICON_BADGE_H