example: add C mouse encoding example

Add a new c-vt-mouse-encode example that demonstrates how to use the
mouse encoder C API. The example creates a mouse encoder configured
with SGR format and normal tracking mode, sets up terminal geometry
for pixel-to-cell coordinate mapping, and encodes a left button press
event into a terminal escape sequence.

Mirrors the structure of the existing c-vt-key-encode example. Also
adds the corresponding @example doxygen reference in vt.h.

example/c-vt-mouse-encode/src/main.c

@@ -0,0 +1,78 @@
+#include <assert.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <string.h>
+#include <ghostty/vt.h>
+
+int main() {
+  GhosttyMouseEncoder encoder;
+  GhosttyResult result = ghostty_mouse_encoder_new(NULL, &encoder);
+  assert(result == GHOSTTY_SUCCESS);
+
+  // Set tracking mode to normal (button press/release)
+  ghostty_mouse_encoder_setopt(encoder, GHOSTTY_MOUSE_ENCODER_OPT_EVENT,
+                               &(GhosttyMouseTrackingMode){GHOSTTY_MOUSE_TRACKING_NORMAL});
+
+  // Set output format to SGR
+  ghostty_mouse_encoder_setopt(encoder, GHOSTTY_MOUSE_ENCODER_OPT_FORMAT,
+                               &(GhosttyMouseFormat){GHOSTTY_MOUSE_FORMAT_SGR});
+
+  // Set terminal geometry so the encoder can map pixel positions to cells
+  ghostty_mouse_encoder_setopt(encoder, GHOSTTY_MOUSE_ENCODER_OPT_SIZE,
+                               &(GhosttyMouseEncoderSize){
+                                   .size = sizeof(GhosttyMouseEncoderSize),
+                                   .screen_width = 800,
+                                   .screen_height = 600,
+                                   .cell_width = 10,
+                                   .cell_height = 20,
+                                   .padding_top = 0,
+                                   .padding_bottom = 0,
+                                   .padding_right = 0,
+                                   .padding_left = 0,
+                               });
+
+  // Create mouse event: left button press at pixel position (50, 40)
+  GhosttyMouseEvent event;
+  result = ghostty_mouse_event_new(NULL, &event);
+  assert(result == GHOSTTY_SUCCESS);
+  ghostty_mouse_event_set_action(event, GHOSTTY_MOUSE_ACTION_PRESS);
+  ghostty_mouse_event_set_button(event, GHOSTTY_MOUSE_BUTTON_LEFT);
+  ghostty_mouse_event_set_position(event, (GhosttyMousePosition){.x = 50.0f, .y = 40.0f});
+  printf("Encoding event: left button press at (50, 40) in SGR format\n");
+
+  // Optionally, encode with null buffer to get required size. You can
+  // skip this step and provide a sufficiently large buffer directly.
+  // If there isn't enough space, the function will return an out of memory
+  // error.
+  size_t required = 0;
+  result = ghostty_mouse_encoder_encode(encoder, event, NULL, 0, &required);
+  assert(result == GHOSTTY_OUT_OF_MEMORY);
+  printf("Required buffer size: %zu bytes\n", required);
+
+  // Encode the mouse event. We don't use our required size above because
+  // that was just an example; we know 128 bytes is enough.
+  char buf[128];
+  size_t written = 0;
+  result = ghostty_mouse_encoder_encode(encoder, event, buf, sizeof(buf), &written);
+  assert(result == GHOSTTY_SUCCESS);
+  printf("Encoded %zu bytes\n", written);
+
+  // Print the encoded sequence (hex and string)
+  printf("Hex: ");
+  for (size_t i = 0; i < written; i++) printf("%02x ", (unsigned char)buf[i]);
+  printf("\n");
+
+  printf("String: ");
+  for (size_t i = 0; i < written; i++) {
+    if (buf[i] == 0x1b) {
+      printf("\\x1b");
+    } else {
+      printf("%c", buf[i]);
+    }
+  }
+  printf("\n");
+
+  ghostty_mouse_event_free(event);
+  ghostty_mouse_encoder_free(encoder);
+  return 0;
+}