vt: add ghostty_type_json for struct layout metadata

Add a new C API function that returns a comptime-generated JSON string
describing the size, alignment, and field layout of every C API extern
struct. This lets FFI consumers (particularly WASM) construct structs
by byte offset without hardcoding platform-specific layout.

The JSON is built at comptime using std.json.Stringify via a
StructInfo type that holds per-struct metadata and implements
jsonStringify. A StaticStringMap keyed by C struct name provides
lookup by name as well as iteration for the JSON serialization.

The function is declared in types.h alongside the other common types
and exported as ghostty_type_json.

include/ghostty/vt/types.h

@@ -83,4 +83,42 @@ typedef struct {
 #define GHOSTTY_INIT_SIZED(type) \
   ((type){ .size = sizeof(type) })
 
+/**
+ * Return a pointer to a JSON string describing the layout of every
+ * C API struct for the current target.
+ *
+ * This is primarily useful for language bindings that can't easily
+ * set C struct fields and need to do so via byte offsets. For example,
+ * WebAssembly modules can't share struct definitions with the host.
+ *
+ * Example (abbreviated):
+ * @code{.json}
+ * {
+ *   "GhosttyMouseEncoderSize": {
+ *     "size": 40,
+ *     "align": 8,
+ *     "fields": {
+ *       "size":           { "offset": 0,  "size": 8, "type": "u64" },
+ *       "screen_width":   { "offset": 8,  "size": 4, "type": "u32" },
+ *       "screen_height":  { "offset": 12, "size": 4, "type": "u32" },
+ *       "cell_width":     { "offset": 16, "size": 4, "type": "u32" },
+ *       "cell_height":    { "offset": 20, "size": 4, "type": "u32" },
+ *       "padding_top":    { "offset": 24, "size": 4, "type": "u32" },
+ *       "padding_bottom": { "offset": 28, "size": 4, "type": "u32" },
+ *       "padding_right":  { "offset": 32, "size": 4, "type": "u32" },
+ *       "padding_left":   { "offset": 36, "size": 4, "type": "u32" }
+ *     }
+ *   }
+ * }
+ * @endcode
+ *
+ * The returned pointer is valid for the lifetime of the process.
+ * The length of the string (excluding any null terminator) is
+ * written to @p len.
+ *
+ * @param[out] len Receives the length of the returned string in bytes.
+ * @return Pointer to the JSON string.
+ */
+const char *ghostty_type_json(size_t *len);
+
 #endif /* GHOSTTY_VT_TYPES_H */