vt: add C API header for terminal mode tags

Add modes.h with GhosttyModeTag, a uint16_t typedef matching the
Zig ModeTag packed struct layout (bits 0-14 for the mode value,
bit 15 for the ANSI flag). Three inline helper functions provide
construction and inspection: ghostty_mode_tag_new,
ghostty_mode_tag_value, and ghostty_mode_tag_ansi.

include/ghostty/vt.h

@@ -99,6 +99,7 @@ extern "C" {
 #include <ghostty/vt/osc.h>
 #include <ghostty/vt/sgr.h>
 #include <ghostty/vt/key.h>
+#include <ghostty/vt/modes.h>
 #include <ghostty/vt/mouse.h>
 #include <ghostty/vt/paste.h>
 #include <ghostty/vt/wasm.h>

include/ghostty/vt/modes.h

@@ -0,0 +1,108 @@
+/**
+ * @file modes.h
+ *
+ * Terminal mode tag utilities - pack and unpack ANSI/DEC mode identifiers.
+ */
+
+#ifndef GHOSTTY_VT_MODES_H
+#define GHOSTTY_VT_MODES_H
+
+/** @defgroup modes Mode Utilities
+ *
+ * Utilities for working with terminal mode tags. A mode tag is a compact
+ * 16-bit representation of a terminal mode identifier that encodes both
+ * the numeric mode value (up to 15 bits) and whether the mode is an ANSI
+ * mode or a DEC private mode (?-prefixed).
+ *
+ * The packed layout (least-significant bit first) is:
+ * - Bits 0–14: mode value (u15)
+ * - Bit 15: ANSI flag (0 = DEC private mode, 1 = ANSI mode)
+ *
+ * ## Example
+ *
+ * @code{.c}
+ * #include <stdio.h>
+ * #include <ghostty/vt.h>
+ *
+ * int main() {
+ *   // Create a tag for DEC mode 25 (cursor visible)
+ *   GhosttyModeTag tag = ghostty_mode_tag_new(25, false);
+ *   printf("value=%u ansi=%d packed=0x%04x\n",
+ *       ghostty_mode_tag_value(tag),
+ *       ghostty_mode_tag_ansi(tag),
+ *       tag);
+ *
+ *   // Create a tag for ANSI mode 4 (insert mode)
+ *   GhosttyModeTag ansi_tag = ghostty_mode_tag_new(4, true);
+ *   printf("value=%u ansi=%d packed=0x%04x\n",
+ *       ghostty_mode_tag_value(ansi_tag),
+ *       ghostty_mode_tag_ansi(ansi_tag),
+ *       ansi_tag);
+ *
+ *   return 0;
+ * }
+ * @endcode
+ *
+ * @{
+ */
+
+#include <stdbool.h>
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * A packed 16-bit terminal mode tag.
+ *
+ * Encodes a mode value (bits 0–14) and an ANSI flag (bit 15) into a
+ * single 16-bit integer. Use the inline helper functions to construct
+ * and inspect mode tags rather than manipulating bits directly.
+ */
+typedef uint16_t GhosttyModeTag;
+
+/**
+ * Create a mode tag from a mode value and ANSI flag.
+ *
+ * @param value The numeric mode value (0–32767)
+ * @param ansi true for an ANSI mode, false for a DEC private mode
+ * @return The packed mode tag
+ *
+ * @ingroup modes
+ */
+static inline GhosttyModeTag ghostty_mode_tag_new(uint16_t value, bool ansi) {
+    return (GhosttyModeTag)((value & 0x7FFF) | ((uint16_t)ansi << 15));
+}
+
+/**
+ * Extract the numeric mode value from a mode tag.
+ *
+ * @param tag The mode tag
+ * @return The mode value (0–32767)
+ *
+ * @ingroup modes
+ */
+static inline uint16_t ghostty_mode_tag_value(GhosttyModeTag tag) {
+    return tag & 0x7FFF;
+}
+
+/**
+ * Check whether a mode tag represents an ANSI mode.
+ *
+ * @param tag The mode tag
+ * @return true if this is an ANSI mode, false if it is a DEC private mode
+ *
+ * @ingroup modes
+ */
+static inline bool ghostty_mode_tag_ansi(GhosttyModeTag tag) {
+    return (tag >> 15) != 0;
+}
+
+#ifdef __cplusplus
+}
+#endif
+
+/** @} */
+
+#endif /* GHOSTTY_VT_MODES_H */