vt: add format_alloc to C API formatter

Rename the existing format function to format_buf to clarify that it
writes into a caller-provided buffer. Add a new format_alloc variant
that allocates the output buffer internally using the provided
allocator (or the default if NULL). The caller receives the allocated
pointer and length and is responsible for freeing it.

This is useful for consumers that do not know the required buffer size
ahead of time and want to avoid the two-pass query-then-format pattern
needed with format_buf.

include/ghostty/vt/formatter.h

@@ -176,10 +176,32 @@ GhosttyResult ghostty_formatter_terminal_new(
  *
  * @ingroup formatter
  */
-GhosttyResult ghostty_formatter_format(GhosttyFormatter formatter,
-                                       uint8_t* buf,
-                                       size_t buf_len,
-                                       size_t* out_written);
+GhosttyResult ghostty_formatter_format_buf(GhosttyFormatter formatter,
+                                           uint8_t* buf,
+                                           size_t buf_len,
+                                           size_t* out_written);
+
+/**
+ * Run the formatter and return an allocated buffer with the output.
+ *
+ * Each call formats the current terminal state. The buffer is allocated
+ * using the provided allocator (or the default allocator if NULL).
+ * The caller is responsible for freeing the returned buffer using the
+ * same allocator.
+ *
+ * @param formatter The formatter handle (must not be NULL)
+ * @param allocator Pointer to allocator, or NULL to use the default allocator
+ * @param out_ptr Pointer to receive the allocated buffer
+ * @param out_len Pointer to receive the length of the output in bytes
+ * @return GHOSTTY_SUCCESS on success, GHOSTTY_OUT_OF_MEMORY on allocation
+ *         failure
+ *
+ * @ingroup formatter
+ */
+GhosttyResult ghostty_formatter_format_alloc(GhosttyFormatter formatter,
+                                             const GhosttyAllocator* allocator,
+                                             uint8_t** out_ptr,
+                                             size_t* out_len);
 
 /**
  * Free a formatter instance.