style(274): retune tunnel, shrine and other colours

Dark palette, chosen while previewing in the Studio:
- tunnel #b6c5c1 -> #a6ccc2
- shrine #a05050 -> #b94141
- other #a7b7d6 -> #8b9ab8

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PLt25nCVMsoroKALYUBu6u

src/song-274/palette.ts

@@ -42,13 +42,13 @@ export const darkPalette: TransectPalette = {
   roadAhead: "#49565f", // --rule
   roadCovered: "#fdb021", // --road
   bridge: "#9be9de", // --bridge
-  tunnel: "#b6c5c1", // --tunnel
+  tunnel: "#a6ccc2", // --tunnel
   shed: "#eba9eb", // --shed
   watercourse: "#5FB6D6", // --water
   place: "#68ad7f", // --place
   railway: "#68ad7f", // --railway
-  shrine: "#a05050", // --shrine
-  other: "#a7b7d6", // --util
+  shrine: "#b94141", // --shrine
+  other: "#8b9ab8", // --util
 };
 
 export const lightPalette: TransectPalette = {