refactor(*): DRY up arena scenes, game modes, and HUD instruments

Arenas inherit from a new arena_base.tscn instead of restating ~40 shared
lines each; only sky/ambient/glow/tint/decoration vary, exposed via new
Arena exports since nested Environment properties aren't overridable
through scene inheritance. Bot construction and score-keeping move onto
GameMode, shared by match and spectate modes while preserving their
differing GameSettings-override behavior and the HUD's score-row
duck-typing. HUD instruments share a HudInstrument base for the
smoothing-weight calc and angle-lerp helper. Also dedupes
MAIN_MENU_SCENE_PATH into ScenePaths and documents why DIFFICULTIES
tiers share one checkpoint.

Game/scripts/hud_heading_tape.gd

@@ -1,5 +1,5 @@
 class_name HudHeadingTape
-extends Control
+extends HudInstrument
 
 # Aircraft-style heading tape: a strip of compass ticks slides under a fixed
 # centre marker, with cardinal letters so the player always knows which way
@@ -11,7 +11,6 @@ const MARKER_COLOR := Color(1.0, 0.8, 0.2)
 
 # Degrees of heading visible across the full tape width.
 const VISIBLE_RANGE := 90.0
-const SMOOTHING := 12.0
 
 const CARDINALS := {0: "N", 45: "NE", 90: "E", 135: "SE", 180: "S", 225: "SW", 270: "W", 315: "NW"}
 
@@ -28,8 +27,8 @@ func set_heading(heading_deg: float) -> void:
 
 
 func _process(delta: float) -> void:
-	var t := 1.0 - exp(-SMOOTHING * delta)  # frame-rate independent
-	_heading = fmod(HudAttitudeIndicator.lerp_angle_deg(_heading, _target_heading, t) + 360.0, 360.0)
+	var t := _smoothing_weight(delta)
+	_heading = fmod(lerp_angle_deg(_heading, _target_heading, t) + 360.0, 360.0)
 	queue_redraw()