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/shaders/arena_deck.gdshader

@@ -21,8 +21,12 @@ uniform vec3 panel_color : source_color = vec3(0.104, 0.116, 0.146);
 uniform float hull_fill : hint_range(0.0, 2.0) = 0.55;
 uniform float panel_variation : hint_range(0.0, 1.0) = 0.4;
 uniform vec3 line_color : source_color = vec3(0.62, 0.78, 1.0);
-uniform vec3 team0_color : source_color = vec3(0.15, 0.45, 1.0);
-uniform vec3 team1_color : source_color = vec3(1.0, 0.35, 0.25);
+// Must track TeamColors.TEAM_COLORS in scripts/team_colors.gd — shaders can't
+// import GDScript consts, and arena_boundary.gd always overrides these at
+// runtime, but keep the literal defaults in sync for anyone inspecting the
+// shader/material directly.
+uniform vec3 team0_color : source_color = vec3(0.48, 0.18, 0.88);
+uniform vec3 team1_color : source_color = vec3(0.24, 0.86, 0.42);
 uniform vec3 seam_color : source_color = vec3(0.35, 0.75, 1.0);
 
 // Play-volume dimensions, from ArenaBoundary's constants.