vt: add style C API

Expose the terminal Style struct to the C API as GhosttyStyle, a
sized struct with foreground, background, and underline colors
(as tagged unions) plus boolean text decoration flags.

Add ghostty_style_default() to obtain the default style and
ghostty_style_is_default() to check whether a style has all
default values. Wire both through c/style.zig, main.zig, and
lib_vt.zig with the corresponding header in vt/style.h.

include/ghostty/vt.h

@@ -98,6 +98,7 @@ extern "C" {
 #include <ghostty/vt/terminal.h>
 #include <ghostty/vt/osc.h>
 #include <ghostty/vt/sgr.h>
+#include <ghostty/vt/style.h>
 #include <ghostty/vt/key.h>
 #include <ghostty/vt/modes.h>
 #include <ghostty/vt/mouse.h>

include/ghostty/vt/style.h

@@ -0,0 +1,128 @@
+/**
+ * @file style.h
+ *
+ * Terminal cell style types.
+ */
+
+#ifndef GHOSTTY_VT_STYLE_H
+#define GHOSTTY_VT_STYLE_H
+
+#include <ghostty/vt/color.h>
+#include <ghostty/vt/types.h>
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/** @defgroup style Style
+ *
+ * Terminal cell style attributes.
+ *
+ * A style describes the visual attributes of a terminal cell, including
+ * foreground, background, and underline colors, as well as flags for
+ * bold, italic, underline, and other text decorations.
+ *
+ * @{
+ */
+
+/**
+ * Style color tags.
+ *
+ * These values identify the type of color in a style color.
+ * Use the tag to determine which field in the color value union to access.
+ *
+ * @ingroup style
+ */
+typedef enum {
+  GHOSTTY_STYLE_COLOR_NONE = 0,
+  GHOSTTY_STYLE_COLOR_PALETTE = 1,
+  GHOSTTY_STYLE_COLOR_RGB = 2,
+} GhosttyStyleColorTag;
+
+/**
+ * Style color value union.
+ *
+ * Use the tag to determine which field is active.
+ *
+ * @ingroup style
+ */
+typedef union {
+  GhosttyColorPaletteIndex palette;
+  GhosttyColorRgb rgb;
+  uint32_t _padding;
+} GhosttyStyleColorValue;
+
+/**
+ * Style color (tagged union).
+ *
+ * A color used in a style attribute. Can be unset (none), a palette
+ * index, or a direct RGB value.
+ *
+ * @ingroup style
+ */
+typedef struct {
+  GhosttyStyleColorTag tag;
+  GhosttyStyleColorValue value;
+} GhosttyStyleColor;
+
+/**
+ * Terminal cell style.
+ *
+ * Describes the complete visual style for a terminal cell, including
+ * foreground, background, and underline colors, as well as text
+ * decoration flags. The underline field uses the same values as
+ * GhosttySgrUnderline.
+ *
+ * This is a sized struct. Use GHOSTTY_INIT_SIZED() to initialize it.
+ *
+ * @ingroup style
+ */
+typedef struct {
+  size_t size;
+  GhosttyStyleColor fg_color;
+  GhosttyStyleColor bg_color;
+  GhosttyStyleColor underline_color;
+  bool bold;
+  bool italic;
+  bool faint;
+  bool blink;
+  bool inverse;
+  bool invisible;
+  bool strikethrough;
+  bool overline;
+  int underline; /**< One of GHOSTTY_SGR_UNDERLINE_* values */
+} GhosttyStyle;
+
+/**
+ * Get the default style.
+ *
+ * Initializes the style to the default values (no colors, no flags).
+ *
+ * @param style Pointer to the style to initialize
+ *
+ * @ingroup style
+ */
+void ghostty_style_default(GhosttyStyle* style);
+
+/**
+ * Check if a style is the default style.
+ *
+ * Returns true if all colors are unset and all flags are off.
+ *
+ * @param style Pointer to the style to check
+ * @return true if the style is the default style
+ *
+ * @ingroup style
+ */
+bool ghostty_style_is_default(const GhosttyStyle* style);
+
+#ifdef __cplusplus
+}
+#endif
+
+/** @} */
+
+#endif /* GHOSTTY_VT_STYLE_H */