fix(s101): centre the M_NSYS direction-of-buoyage arrow on its area

DIRBOY01/A1/B1 carry their catalogue pivot at the arrow tail (SVG 0,0,
bottom-centre), so anchoring the pivot at the region centre pushed the whole
arrow ~30 mm north and out the top of the small demo regions. Set CentreOnArea
so the client centres the glyph on the representative point instead (S-52
§8.5.1), keeping the arrow inside its box.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: beetlebugorg

internal/engine/portrayal/build.go

@@ -705,9 +705,13 @@ func navSystemBuild(f *s57.Feature) FeatureBuild {
 			arrow = "DIRBOYB1"
 		}
 		if a, ok := areaSurfacePoint(coordsToLatLon(exteriorRing(g))); ok {
+			// Centre the arrow on the area's representative point: DIRBOY's catalogue
+			// pivot sits at the arrow's tail (SVG 0,0, bottom-centre), so honouring it
+			// would push the whole arrow ~30 mm north of the centre and out of a small
+			// region. CentreOnArea centres the glyph instead (S-52 §8.5.1).
 			prims = append(prims, SymbolCall{
 				Anchor: a, SymbolName: arrow, RotationDeg: float32(orient), RotationTrueNorth: true,
-				Scale: DefaultPxPerSymbolUnit, SoundingDepthM: nan32, DangerDepthM: nan32,
+				CentreOnArea: true, Scale: DefaultPxPerSymbolUnit, SoundingDepthM: nan32, DangerDepthM: nan32,
 			})
 		}
 	}