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>

src/lib_vt.zig

@@ -218,6 +218,7 @@ comptime {
         @export(&c.grid_ref_graphemes, .{ .name = "ghostty_grid_ref_graphemes" });
         @export(&c.grid_ref_style, .{ .name = "ghostty_grid_ref_style" });
         @export(&c.build_info, .{ .name = "ghostty_build_info" });
+        @export(&c.free_alloc, .{ .name = "ghostty_free" });
 
         // On Wasm we need to export our allocator convenience functions.
         if (builtin.target.cpu.arch.isWasm()) {

src/terminal/c/main.zig

@@ -1,3 +1,6 @@
+const lib_alloc = @import("../../lib/allocator.zig");
+const CAllocator = lib_alloc.Allocator;
+
 const buildpkg = @import("build_info.zig");
 pub const cell = @import("cell.zig");
 pub const color = @import("color.zig");
@@ -112,6 +115,21 @@ pub const mouse_encoder_encode = mouse_encode.encode;
 
 pub const paste_is_safe = paste.is_safe;
 
+/// Free memory that was allocated by a libghostty-vt function.
+///
+/// This must be used to free buffers returned by functions like
+/// `format_alloc`. Pass the same allocator (or NULL for the default)
+/// that was used for the allocation.
+pub fn free_alloc(
+    alloc_: ?*const CAllocator,
+    ptr: ?[*]u8,
+    len: usize,
+) callconv(.c) void {
+    const mem = ptr orelse return;
+    const alloc = lib_alloc.default(alloc_);
+    alloc.free(mem[0..len]);
+}
+
 pub const size_report_encode = size_report.encode;
 
 pub const cell_get = cell.get;