Implement unified tunnel build mode (REQ-BLD-TUNNEL-MODE)

Collapse the two tunnel buttons into a single Tunnel button whose ghost
resolves between Tunnel Entry and Tunnel Exit based on the hovered
position, with a green connection preview.

- Add pure core helper TunnelCompletion (firstTunnelFacing +
  resolveTunnelCompletion): the entry/exit completion scans and the
  sub-tile cursor tie-break, unit-tested in TunnelCompletionTest.
- Dedup BeltSystem::reevaluateTunnelPairing onto firstTunnelFacing so the
  pairing scan rule lives in one place.
- GameWorldView: resolve the ghost type on hover/rotate (matching built
  tunnels and construction sites), place the resolved type (drop the old
  auto-switch), and draw the matched partner + between-tiles in green.
  Hotkey 3 = Tunnel, 4 unused.
- BuildButtonGrid: one "Tunnel" button; tunnel_exit stays player-placeable
  so blueprints and cost totals still account for exits.
- Add the green tunnel_preview overlay color (VisualsConfig/Loader/toml).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y7N59FsLA5e2kuVdqe4Uhc

src/test/CMakeLists.txt

@@ -9,6 +9,7 @@ add_files(
     BeltSystemTest.cpp
     SurfaceMaskTest.cpp
     BeltDragPathTest.cpp
+    TunnelCompletionTest.cpp
     BuildingTest.cpp
     BuildingConfigTest.cpp
     ShipTest.cpp