libghostty: add ghostty_free for cross-runtime memory safety

On Windows, Zig's built-in libc and MSVC's CRT maintain separate
heaps, so calling free() on memory allocated by the library causes
undefined behavior. Add ghostty_free() that frees through the same
allocator that performed the allocation, making it safe on all
platforms.

Update format_alloc docs and all examples to use ghostty_free()
instead of free().

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

example/c-vt-cmake-static/src/main.c

@@ -45,7 +45,7 @@ int main() {
   fwrite(buf, 1, len, stdout);
   printf("\n");
 
-  free(buf);
+  ghostty_free(NULL, buf, len);
   ghostty_formatter_free(formatter);
   ghostty_terminal_free(terminal);
   return 0;

example/c-vt-cmake/src/main.c

@@ -45,7 +45,7 @@ int main() {
   fwrite(buf, 1, len, stdout);
   printf("\n");
 
-  free(buf);
+  ghostty_free(NULL, buf, len);
   ghostty_formatter_free(formatter);
   ghostty_terminal_free(terminal);
   return 0;

example/c-vt-formatter/src/main.c

@@ -56,7 +56,7 @@ int main() {
   printf("\n");
 
   // Clean up
-  free(buf);
+  ghostty_free(NULL, buf, len);
   ghostty_formatter_free(formatter);
   ghostty_terminal_free(terminal);
   return 0;