feat(transit): give vehicle icons a readable front, and none without a course

At the ~20 px the fleet renders at, the bus, tram, metro and unknown
glyphs were near-symmetric boxes: rotated along the course, nothing showed
which end was the front. Each now has a front built into its outline (an
arrow-like nose for the bus, a pointed cab for the tram, a chamfered cab
for the metro), so it survives the tint, the halo and the sensor profiles,
which keep only the outline.

A vehicle with no known course sits at rotation 0, where a pointed glyph
would claim it is heading north. Those vehicles, fleet and selected alike,
now get a symmetric outline (rail and ferry included) and switch to the
pointed one once a course arrives. The raster cache doubles to a fixed 72
variants.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

scripts/qa-transit.mjs

@@ -801,7 +801,7 @@ try {
           );
         check(
           `${preset.name}: the icon raster cache stays bounded`,
-          styling.iconCacheSize <= 6 * 2 * 3,
+          styling.iconCacheSize <= 6 * 2 * 3 * 2,
           `${styling.iconCacheSize} rasters`,
         );
       }