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

bin/app/data/config/buildings.toml

@@ -16,7 +16,7 @@ surface_mask = ["<A>"]
 
 [[building]]
 id = "tunnel_entry"
-tooltip = "Sends items underground to a matching tunnel exit, letting belts cross."
+tooltip = "Sends items underground so belts can cross. Places an entry, or an exit when it would connect to a matching entry under the cursor."
 cost = 5
 player_placeable = true
 construction_time_seconds = 0.5

bin/app/data/config/visuals.toml

@@ -351,6 +351,7 @@ selected_outline = "#ffff00"   # outline drawn around currently-selected buildin
 copy_config    = "#33ccff66"   # copy-settings eligible-target tint + copy/paste flash (REQ-BLD-COPY-CONFIG-FEEDBACK)
 locked_asteroid = "#0000007f"  # tint over the asteroid left of the buildable edge (not yet unlocked by expansion)
 modal_dim      = "#00000099"   # semi-transparent black dim behind modal dialogs/menus (REQ-UI-MODAL-DIM)
+tunnel_preview = "#00ff0055"   # tunnel connection preview: matched end + tiles between (REQ-BLD-TUNNEL-MODE)
 
 # -----------------------------------------------------------------------------
 # Schematic-drop toasts (REQ-UI-SCHEMATIC-TOAST)