Add visible terminal control debug helpers

src/terminal_debug.zig

@@ -0,0 +1,36 @@
+const std = @import("std");
+
+/// Materialize terminal control characters as visible glyphs for failure output.
+/// This is test/debug support only: product rendering still writes real CR/LF/ESC.
+pub fn visibleControlsAlloc(allocator: std.mem.Allocator, bytes: []const u8) ![]u8 {
+    var out = std.ArrayList(u8).empty;
+    errdefer out.deinit(allocator);
+    for (bytes) |byte| switch (byte) {
+        '\r' => try out.appendSlice(allocator, "␍"),
+        '\n' => try out.appendSlice(allocator, "␊\n"),
+        0x1b => try out.appendSlice(allocator, "␛"),
+        '\t' => try out.appendSlice(allocator, "⇥"),
+        else => if (byte < 0x20) {
+            try out.appendSlice(allocator, "␀");
+        } else try out.append(allocator, byte),
+    };
+    return out.toOwnedSlice(allocator);
+}
+
+pub fn hasBareLf(bytes: []const u8) bool {
+    for (bytes, 0..) |byte, i| {
+        if (byte == '\n' and (i == 0 or bytes[i - 1] != '\r')) return true;
+    }
+    return false;
+}
+
+test "regular: visible terminal controls expose newline and escape bytes" {
+    const debug = try visibleControlsAlloc(std.testing.allocator, "a\nb\r\n\x1b[31m");
+    defer std.testing.allocator.free(debug);
+    try std.testing.expectEqualStrings("a␊\nb␍␊\n␛[31m", debug);
+}
+
+test "adversarial: bare LF detector catches raw-mode skew risk" {
+    try std.testing.expect(hasBareLf("a\nb"));
+    try std.testing.expect(!hasBareLf("a\r\nb"));
+}