remove the dead ColumnHat widget; keep hatHeight (T-385)

ColumnHat was superseded by the hat bar in app.dart and survived only
through a zero-coverage smoke test. Its file also carried the live
hatHeight constant (D-57's 24px hats) consumed by the hat bar and the
menu bar — that moves to widgets/src/chrome_metrics.dart.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/widgets/src/chrome_metrics.dart

@@ -0,0 +1,11 @@
+/// Shared window-chrome metrics.
+///
+/// `hatHeight` used to live in clide_column_hat.dart; the per-column
+/// `ColumnHat` widget there was dead (duplicated by the hat bar in
+/// app.dart, kept alive only by a zero-coverage test) and was removed
+/// in the T-385 sweep — the constant is the part the live chrome
+/// (app.dart hat bar, menu bar) actually consumes (D-57).
+library;
+
+/// Height of the per-column 24px window hats (D-57).
+const double hatHeight = 24;