vt: expose size_report encoding in the C API

Add ghostty_size_report_encode() to libghostty-vt, following the
same pattern as focus encoding: a single stateless function that
writes a terminal size report escape sequence into a caller-provided
buffer.

The size_report.zig Style enum and Size struct now use lib.Enum and
lib.Struct so the types are automatically C-compatible when building
with c_abi, eliminating the need for duplicate type definitions in
the C wrapper. The C wrapper in c/size_report.zig re-exports these
types directly and provides the callconv(.c) encode entry point.

Supports mode 2048 in-band reports and XTWINOPS responses (CSI 14 t,
CSI 16 t, CSI 18 t).

include/ghostty/vt.h

@@ -102,6 +102,7 @@ extern "C" {
 #include <ghostty/vt/modes.h>
 #include <ghostty/vt/mouse.h>
 #include <ghostty/vt/paste.h>
+#include <ghostty/vt/size_report.h>
 #include <ghostty/vt/wasm.h>
 
 #ifdef __cplusplus

include/ghostty/vt/size_report.h

@@ -0,0 +1,126 @@
+/**
+ * @file size_report.h
+ *
+ * Size report encoding - encode terminal size reports into escape sequences.
+ */
+
+#ifndef GHOSTTY_VT_SIZE_REPORT_H
+#define GHOSTTY_VT_SIZE_REPORT_H
+
+/** @defgroup size_report Size Report Encoding
+ *
+ * Utilities for encoding terminal size reports into escape sequences,
+ * supporting in-band size reports (mode 2048) and XTWINOPS responses
+ * (CSI 14 t, CSI 16 t, CSI 18 t).
+ *
+ * ## Basic Usage
+ *
+ * Use ghostty_size_report_encode() to encode a size 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
+ *
+ * @code{.c}
+ * #include <stdio.h>
+ * #include <ghostty/vt.h>
+ *
+ * int main() {
+ *   GhosttySizeReportSize size = {
+ *     .rows = 24,
+ *     .columns = 80,
+ *     .cell_width = 9,
+ *     .cell_height = 18,
+ *   };
+ *
+ *   char buf[64];
+ *   size_t written = 0;
+ *
+ *   GhosttyResult result = ghostty_size_report_encode(
+ *       GHOSTTY_SIZE_REPORT_MODE_2048, size, buf, sizeof(buf), &written);
+ *
+ *   if (result == GHOSTTY_SUCCESS) {
+ *     printf("Encoded %zu bytes: ", written);
+ *     fwrite(buf, 1, written, stdout);
+ *     printf("\n");
+ *   }
+ *
+ *   return 0;
+ * }
+ * @endcode
+ *
+ * @{
+ */
+
+#include <stddef.h>
+#include <stdint.h>
+#include <ghostty/vt/types.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * Size report style.
+ *
+ * Determines the output format for the terminal size report.
+ */
+typedef enum {
+    /** In-band size report (mode 2048): ESC [ 48 ; rows ; cols ; height ; width t */
+    GHOSTTY_SIZE_REPORT_MODE_2048 = 0,
+    /** XTWINOPS text area size in pixels: ESC [ 4 ; height ; width t */
+    GHOSTTY_SIZE_REPORT_CSI_14_T = 1,
+    /** XTWINOPS cell size in pixels: ESC [ 6 ; height ; width t */
+    GHOSTTY_SIZE_REPORT_CSI_16_T = 2,
+    /** XTWINOPS text area size in characters: ESC [ 8 ; rows ; cols t */
+    GHOSTTY_SIZE_REPORT_CSI_18_T = 3,
+} GhosttySizeReportStyle;
+
+/**
+ * Terminal size information for encoding size reports.
+ */
+typedef struct {
+    /** Terminal row count in cells. */
+    uint16_t rows;
+    /** Terminal column count in cells. */
+    uint16_t columns;
+    /** Width of a single terminal cell in pixels. */
+    uint32_t cell_width;
+    /** Height of a single terminal cell in pixels. */
+    uint32_t cell_height;
+} GhosttySizeReportSize;
+
+/**
+ * Encode a terminal size report into an escape sequence.
+ *
+ * Encodes a size report in the format specified by @p style into the
+ * provided buffer.
+ *
+ * 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 style The size report format to encode
+ * @param size Terminal size information
+ * @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
+ */
+GhosttyResult ghostty_size_report_encode(
+    GhosttySizeReportStyle style,
+    GhosttySizeReportSize size,
+    char* buf,
+    size_t buf_len,
+    size_t* out_written);
+
+#ifdef __cplusplus
+}
+#endif
+
+/** @} */
+
+#endif /* GHOSTTY_VT_SIZE_REPORT_H */