refactor(team-colors): collapse disagreeing team palettes into one source of truth

ship.gd, HUDController.gd, and goal.gd each declared their own TEAM_COLORS,
arena_boundary.gd/arena_deck.gdshader had a third pair, and HUD.tscn baked
in a fourth (hardcoded "BLUE"/"ORANGE" labels) — nose, goal rim, end zone,
and scoreboard all rendered different blues. New scripts/team_colors.gd
(class_name TeamColors) is now the single source every one of those reads
from, and team identity moves to purple/green.

Game/scripts/team_colors.gd

@@ -0,0 +1,10 @@
+class_name TeamColors
+
+# Single source of truth for team identity. Ship nose/tailfin, HUD result
+# panel, goal rim/net, and the arena end-zone floor shader all read from
+# here — do not restate these values locally (see TODO.md history).
+const TEAM_NAMES := {0: "Purple", 1: "Green"}
+const TEAM_COLORS := {
+	0: Color(0.48, 0.18, 0.88),
+	1: Color(0.24, 0.86, 0.42),
+}