lib-vt: add color scheme report encoder

Add a shared encoder for CSI ? 997 ; Ps n color scheme reports and use 
it for both CSI ? 996 n replies and unsolicited Termio reports. Export the 
same encoder through the libghostty-vt C API with docs and an example.

This is a really light API, arguably easy for consumers to hardcode,
but it didn't match the rest of our style in the libghostty API so we 
should expose it.

Example: GHOSTTY_COLOR_SCHEME_DARK encodes to ESC [ ? 997 ; 1 n,
while GHOSTTY_COLOR_SCHEME_LIGHT encodes to ESC [ ? 997 ; 2 n.

include/ghostty/vt.h

@@ -126,6 +126,7 @@ extern "C" {
 #include <ghostty/vt/allocator.h>
 #include <ghostty/vt/build_info.h>
 #include <ghostty/vt/color.h>
+#include <ghostty/vt/color_scheme.h>
 #include <ghostty/vt/device.h>
 #include <ghostty/vt/focus.h>
 #include <ghostty/vt/formatter.h>

include/ghostty/vt/color_scheme.h

@@ -0,0 +1,73 @@
+/**
+ * @file color_scheme.h
+ *
+ * Color scheme report encoding - encode terminal color scheme reports into
+ * escape sequences.
+ */
+
+#ifndef GHOSTTY_VT_COLOR_SCHEME_H
+#define GHOSTTY_VT_COLOR_SCHEME_H
+
+/** @defgroup color_scheme Color Scheme Report Encoding
+ *
+ * Utilities for encoding color scheme reports into terminal escape
+ * sequences for color scheme reporting mode (mode 2031).
+ *
+ * ## Basic Usage
+ *
+ * Use ghostty_color_scheme_report_encode() to encode a color scheme report
+ * into a caller-provided buffer. If the buffer is too small, the function
+ * returns GHOSTTY_OUT_OF_SPACE and sets the required size in the output
+ * parameter.
+ *
+ * ## Example
+ *
+ * @snippet c-vt-color-scheme/src/main.c color-scheme-report-encode
+ *
+ * @{
+ */
+
+#include <stddef.h>
+#include <ghostty/vt/types.h>
+#include <ghostty/vt/device.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * Encode a color scheme report into an escape sequence.
+ *
+ * Encodes a color scheme report into the provided buffer. Dark color schemes
+ * emit ESC [ ? 997 ; 1 n, and light color schemes emit ESC [ ? 997 ; 2 n.
+ * The encoded bytes are identical to the terminal's internal CSI ? 996 n
+ * query response.
+ *
+ * Hosts should gate unsolicited sends on GHOSTTY_MODE_COLOR_SCHEME_REPORT
+ * (mode 2031) being set, which can be checked via the mode getters.
+ *
+ * If the buffer is too small, the function returns GHOSTTY_OUT_OF_SPACE
+ * and writes the required buffer size to @p out_written. The caller can
+ * then retry with a sufficiently sized buffer.
+ *
+ * @param scheme The color scheme to encode
+ * @param buf Output buffer to write the encoded sequence into (may be NULL)
+ * @param buf_len Size of the output buffer in bytes
+ * @param[out] out_written On success, the number of bytes written. On
+ *             GHOSTTY_OUT_OF_SPACE, the required buffer size.
+ * @return GHOSTTY_SUCCESS on success, GHOSTTY_OUT_OF_SPACE if the buffer
+ *         is too small
+ */
+GHOSTTY_API GhosttyResult ghostty_color_scheme_report_encode(
+    GhosttyColorScheme scheme,
+    char* buf,
+    size_t buf_len,
+    size_t* out_written);
+
+#ifdef __cplusplus
+}
+#endif
+
+/** @} */
+
+#endif /* GHOSTTY_VT_COLOR_SCHEME_H */