Do not send unsolicited DSR 997 on mode 2031 enable

DECSET 2031 (ColorPaletteUpdates) should notify applications only when
the color scheme changes, not immediately on enable. The unsolicited
initial report leaks into applications that enable the mode without
arranging to consume a response, surfacing as literal `?997;1n` text
in the shell input buffer.

Mirrors the fix applied to DECSET 1004 focus reporting in #40.

Fixes manaflow-ai/cmux#2636

Author: okuuva

src/termio/stream_handler.zig

@@ -797,6 +797,16 @@ pub const StreamHandler = struct {
             .mouse_format_urxvt => self.terminal.flags.mouse_format = if (enabled) .urxvt else .x10,
             .mouse_format_sgr_pixels => self.terminal.flags.mouse_format = if (enabled) .sgr_pixels else .x10,
 
+            // Mode 2031 (ColorPaletteUpdates) notifies applications when the
+            // color scheme *changes*. Do not synthesize an immediate DSR 997
+            // report when the mode is enabled, otherwise applications that
+            // enable DECSET 2031 during startup without arranging to consume
+            // a response (for example, Claude Code) receive a spurious CSI
+            // sequence that leaks into the shell input buffer as literal
+            // text. Applications that want the current scheme can query it
+            // explicitly via DSR 996.
+            .report_color_scheme => {},
+
             else => {},
         }
     }
@@ -1554,3 +1564,51 @@ pub const StreamHandler = struct {
         self.surfaceMessageWriter(.{ .progress_report = report });
     }
 };
+
+test "enabling mode 2031 does not emit an immediate color scheme report" {
+    const testing = std.testing;
+
+    var term = try terminal.Terminal.init(testing.allocator, .{
+        .cols = 10,
+        .rows = 5,
+    });
+    defer term.deinit(testing.allocator);
+
+    var mailbox = try termio.Mailbox.initSPSC(testing.allocator);
+    defer mailbox.deinit(testing.allocator);
+
+    var mutex: std.Thread.Mutex = .{};
+    var renderer_state: renderer.State = .{
+        .mutex = &mutex,
+        .terminal = &term,
+    };
+    var size: renderer.Size = .{
+        .screen = .{ .width = 800, .height = 600 },
+        .cell = .{ .width = 8, .height = 16 },
+        .padding = .{},
+    };
+    var handler = StreamHandler{
+        .alloc = testing.allocator,
+        .size = &size,
+        .terminal = &term,
+        .termio_mailbox = &mailbox,
+        .surface_mailbox = undefined,
+        .renderer_state = &renderer_state,
+        .renderer_mailbox = undefined,
+        .renderer_wakeup = undefined,
+        .default_cursor_style = .block,
+        .default_cursor_blink = null,
+        .enquiry_response = "",
+        .osc_color_report_format = .none,
+        .clipboard_write = .deny,
+    };
+    defer handler.deinit();
+
+    mutex.lock();
+    defer mutex.unlock();
+    try handler.setMode(.report_color_scheme, true);
+
+    try testing.expect(term.modes.get(.report_color_scheme));
+    try testing.expect(!handler.termio_messaged);
+    try testing.expect(mailbox.spsc.queue.pop() == null);
+}