vt: expose focus encoding in C and Zig APIs

Add focus event encoding (CSI I / CSI O) to the libghostty-vt public
API, following the same patterns as key and mouse encoding.

The focus Event enum uses lib.Enum for C ABI compatibility. The C API
provides ghostty_focus_encode() which writes into a caller-provided
buffer and returns GHOSTTY_OUT_OF_SPACE with the required size when
the buffer is too small.

Also update key and mouse encoders to return GHOSTTY_OUT_OF_SPACE
instead of GHOSTTY_OUT_OF_MEMORY for buffer-too-small errors,
reserving OUT_OF_MEMORY for actual allocation failures. Update all
corresponding header documentation.

src/lib_vt.zig

@@ -81,11 +81,17 @@ pub const input = struct {
     // We have to be careful to only import targeted files within
     // the input package because the full package brings in too many
     // other dependencies.
+    const focus = terminal.focus;
     const paste = @import("input/paste.zig");
     const key = @import("input/key.zig");
     const key_encode = @import("input/key_encode.zig");
     const mouse_encode = @import("input/mouse_encode.zig");
 
+    // Focus-related APIs
+    pub const max_focus_encode_size = focus.max_encode_size;
+    pub const FocusEvent = focus.Event;
+    pub const encodeFocus = focus.encode;
+
     // Paste-related APIs
     pub const PasteError = paste.Error;
     pub const PasteOptions = paste.Options;
@@ -158,6 +164,7 @@ comptime {
         @export(&c.osc_end, .{ .name = "ghostty_osc_end" });
         @export(&c.osc_command_type, .{ .name = "ghostty_osc_command_type" });
         @export(&c.osc_command_data, .{ .name = "ghostty_osc_command_data" });
+        @export(&c.focus_encode, .{ .name = "ghostty_focus_encode" });
         @export(&c.paste_is_safe, .{ .name = "ghostty_paste_is_safe" });
         @export(&c.color_rgb_get, .{ .name = "ghostty_color_rgb_get" });
         @export(&c.sgr_new, .{ .name = "ghostty_sgr_new" });