vt: expose terminal modes to C API

Add modes.h with GhosttyModeTag (uint16_t) matching the Zig ModeTag
packed struct layout, along with inline helpers for constructing and
inspecting mode tags. Provide GHOSTTY_MODE_* macros for all 39
built-in modes (4 ANSI, 35 DEC), parenthesized for safety.

Add ghostty_terminal_mode_get and ghostty_terminal_mode_set to
terminal.h, both returning GhosttyResult so that null terminals
and unknown mode tags return GHOSTTY_INVALID_VALUE. The get function
writes its result through a bool out-parameter.

Add a note in the Zig mode entries reminding developers to update
modes.h when adding new modes.

src/lib_vt.zig

@@ -186,6 +186,8 @@ comptime {
         @export(&c.terminal_resize, .{ .name = "ghostty_terminal_resize" });
         @export(&c.terminal_vt_write, .{ .name = "ghostty_terminal_vt_write" });
         @export(&c.terminal_scroll_viewport, .{ .name = "ghostty_terminal_scroll_viewport" });
+        @export(&c.terminal_mode_get, .{ .name = "ghostty_terminal_mode_get" });
+        @export(&c.terminal_mode_set, .{ .name = "ghostty_terminal_mode_set" });
 
         // On Wasm we need to export our allocator convenience functions.
         if (builtin.target.cpu.arch.isWasm()) {

src/terminal/c/main.zig

@@ -90,6 +90,8 @@ pub const terminal_reset = terminal.reset;
 pub const terminal_resize = terminal.resize;
 pub const terminal_vt_write = terminal.vt_write;
 pub const terminal_scroll_viewport = terminal.scroll_viewport;
+pub const terminal_mode_get = terminal.mode_get;
+pub const terminal_mode_set = terminal.mode_set;
 
 test {
     _ = color;

src/terminal/c/terminal.zig

@@ -3,6 +3,7 @@ const testing = std.testing;
 const lib_alloc = @import("../../lib/allocator.zig");
 const CAllocator = lib_alloc.Allocator;
 const ZigTerminal = @import("../Terminal.zig");
+const modes = @import("../modes.zig");
 const size = @import("../size.zig");
 const Result = @import("result.zig").Result;
 
@@ -98,6 +99,30 @@ pub fn reset(terminal_: Terminal) callconv(.c) void {
     t.fullReset();
 }
 
+pub fn mode_get(
+    terminal_: Terminal,
+    tag: modes.ModeTag.Backing,
+    out_value: *bool,
+) callconv(.c) Result {
+    const t = terminal_ orelse return .invalid_value;
+    const mode_tag: modes.ModeTag = @bitCast(tag);
+    const mode = modes.modeFromInt(mode_tag.value, mode_tag.ansi) orelse return .invalid_value;
+    out_value.* = t.modes.get(mode);
+    return .success;
+}
+
+pub fn mode_set(
+    terminal_: Terminal,
+    tag: modes.ModeTag.Backing,
+    value: bool,
+) callconv(.c) Result {
+    const t = terminal_ orelse return .invalid_value;
+    const mode_tag: modes.ModeTag = @bitCast(tag);
+    const mode = modes.modeFromInt(mode_tag.value, mode_tag.ansi) orelse return .invalid_value;
+    t.modes.set(mode, value);
+    return .success;
+}
+
 pub fn free(terminal_: Terminal) callconv(.c) void {
     const t = terminal_ orelse return;
 
@@ -272,6 +297,87 @@ test "resize invalid value" {
     try testing.expectEqual(Result.invalid_value, resize(t, 80, 0));
 }
 
+test "mode_get and mode_set" {
+    var t: Terminal = null;
+    try testing.expectEqual(Result.success, new(
+        &lib_alloc.test_allocator,
+        &t,
+        .{
+            .cols = 80,
+            .rows = 24,
+            .max_scrollback = 0,
+        },
+    ));
+    defer free(t);
+
+    var value: bool = undefined;
+
+    // DEC mode 25 (cursor_visible) defaults to true
+    const cursor_visible: modes.ModeTag.Backing = @bitCast(modes.ModeTag{ .value = 25, .ansi = false });
+    try testing.expectEqual(Result.success, mode_get(t, cursor_visible, &value));
+    try testing.expect(value);
+
+    // Set it to false
+    try testing.expectEqual(Result.success, mode_set(t, cursor_visible, false));
+    try testing.expectEqual(Result.success, mode_get(t, cursor_visible, &value));
+    try testing.expect(!value);
+
+    // ANSI mode 4 (insert) defaults to false
+    const insert: modes.ModeTag.Backing = @bitCast(modes.ModeTag{ .value = 4, .ansi = true });
+    try testing.expectEqual(Result.success, mode_get(t, insert, &value));
+    try testing.expect(!value);
+
+    try testing.expectEqual(Result.success, mode_set(t, insert, true));
+    try testing.expectEqual(Result.success, mode_get(t, insert, &value));
+    try testing.expect(value);
+}
+
+test "mode_get null" {
+    var value: bool = undefined;
+    const tag: modes.ModeTag.Backing = @bitCast(modes.ModeTag{ .value = 25, .ansi = false });
+    try testing.expectEqual(Result.invalid_value, mode_get(null, tag, &value));
+}
+
+test "mode_set null" {
+    const tag: modes.ModeTag.Backing = @bitCast(modes.ModeTag{ .value = 25, .ansi = false });
+    try testing.expectEqual(Result.invalid_value, mode_set(null, tag, true));
+}
+
+test "mode_get unknown mode" {
+    var t: Terminal = null;
+    try testing.expectEqual(Result.success, new(
+        &lib_alloc.test_allocator,
+        &t,
+        .{
+            .cols = 80,
+            .rows = 24,
+            .max_scrollback = 0,
+        },
+    ));
+    defer free(t);
+
+    var value: bool = undefined;
+    const unknown: modes.ModeTag.Backing = @bitCast(modes.ModeTag{ .value = 9999, .ansi = false });
+    try testing.expectEqual(Result.invalid_value, mode_get(t, unknown, &value));
+}
+
+test "mode_set unknown mode" {
+    var t: Terminal = null;
+    try testing.expectEqual(Result.success, new(
+        &lib_alloc.test_allocator,
+        &t,
+        .{
+            .cols = 80,
+            .rows = 24,
+            .max_scrollback = 0,
+        },
+    ));
+    defer free(t);
+
+    const unknown: modes.ModeTag.Backing = @bitCast(modes.ModeTag{ .value = 9999, .ansi = false });
+    try testing.expectEqual(Result.invalid_value, mode_set(t, unknown, true));
+}
+
 test "vt_write" {
     var t: Terminal = null;
     try testing.expectEqual(Result.success, new(

src/terminal/modes.zig

@@ -245,6 +245,9 @@ const ModeEntry = struct {
 /// The full list of available entries. For documentation see how
 /// they're used within Ghostty or google their values. It is not
 /// valuable to redocument them all here.
+///
+/// NOTE: When adding a new mode entry, also add a corresponding
+/// GHOSTTY_MODE_* macro in include/ghostty/vt/modes.h.
 const entries: []const ModeEntry = &.{
     // ANSI
     .{ .name = "disable_keyboard", .value = 2, .ansi = true }, // KAM