vt: move free_alloc to dedicated allocator.zig

Extract the inline free_alloc function from main.zig into a new
allocator.zig module in the C API layer. The function is renamed
to alloc_free in main.zig (and free in allocator.zig) for
consistency with the other C API naming conventions. Add tests
for null pointer, allocated memory, and null allocator fallback.

src/lib_vt.zig

@@ -218,7 +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" });
+        @export(&c.alloc_free, .{ .name = "ghostty_free" });
 
         // On Wasm we need to export our allocator convenience functions.
         if (builtin.target.cpu.arch.isWasm()) {

src/terminal/c/allocator.zig

@@ -0,0 +1,36 @@
+const std = @import("std");
+const testing = std.testing;
+const lib_alloc = @import("../../lib/allocator.zig");
+const CAllocator = lib_alloc.Allocator;
+
+/// 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_: ?*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]);
+}
+
+test "free null pointer" {
+    free(&lib_alloc.test_allocator, null, 0);
+}
+
+test "free allocated memory" {
+    const alloc = lib_alloc.default(&lib_alloc.test_allocator);
+    const mem = try alloc.alloc(u8, 16);
+    free(&lib_alloc.test_allocator, mem.ptr, mem.len);
+}
+
+test "free with null allocator" {
+    // null allocator falls back to the default (test allocator in tests)
+    const alloc = lib_alloc.default(null);
+    const mem = try alloc.alloc(u8, 8);
+    free(null, mem.ptr, mem.len);
+}

src/terminal/c/main.zig

@@ -2,6 +2,7 @@ const lib_alloc = @import("../../lib/allocator.zig");
 const CAllocator = lib_alloc.Allocator;
 
 const buildpkg = @import("build_info.zig");
+pub const allocator = @import("allocator.zig");
 pub const cell = @import("cell.zig");
 pub const color = @import("color.zig");
 pub const focus = @import("focus.zig");
@@ -115,20 +116,7 @@ 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 alloc_free = allocator.free;
 
 pub const size_report_encode = size_report.encode;
 
@@ -157,6 +145,7 @@ pub const grid_ref_graphemes = grid_ref.grid_ref_graphemes;
 pub const grid_ref_style = grid_ref.grid_ref_style;
 
 test {
+    _ = allocator;
     _ = buildpkg;
     _ = cell;
     _ = color;