show the tunnel end the click would actually place

The controls panel header read the type builder mode was entered with, so
tunnel mode always said Tunnel Entry even where the ghost had resolved to an
exit. Feed the header the same effective type the ghost and placement already
use.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ne3mejABZoLWKLh8fgpM3x

src/lib/core/ControlAction.h

@@ -109,7 +109,10 @@ enum class ControlContextKind
 struct ControlContext
 {
     BuildMode        mode                      = BuildMode::None;
-    BuildingType     builderType               = BuildingType::Belt;  // while mode == Builder
+    // While mode == Builder: the type a click would place at the current hover position,
+    // not the type the mode was entered with — tunnel mode resolves to either end
+    // (REQ-BLD-TUNNEL-MODE).
+    BuildingType     builderType               = BuildingType::Belt;
     bool             draggingBelt              = false;
     // A single-building blueprint whose ghost is over a configuration-transfer target,
     // so clicking hands over settings rather than placing (REQ-UI-BLUEPRINT-TRANSFER).