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_attitude_indicator.gd

@@ -1,5 +1,5 @@
 class_name HudAttitudeIndicator
-extends Control
+extends HudInstrument
 
 # Aircraft-style attitude indicator (artificial horizon): the sky/ground disc
 # rolls and the pitch ladder scrolls behind a fixed ship symbol, so pitch and
@@ -14,8 +14,6 @@ const RING_COLOR := Color(0.5, 0.85, 1.0, 0.7)
 
 # Instrument scale: degrees of pitch visible from centre to rim.
 const PITCH_RANGE := 45.0
-# Display smoothing rate (per second); high = snappy, low = floaty.
-const SMOOTHING := 12.0
 
 var _target_pitch := 0.0
 var _target_roll := 0.0
@@ -29,16 +27,12 @@ func set_attitude(pitch_deg: float, roll_deg: float) -> void:
 
 
 func _process(delta: float) -> void:
-	var t := 1.0 - exp(-SMOOTHING * delta)  # frame-rate independent
+	var t := _smoothing_weight(delta)
 	_pitch = lerpf(_pitch, _target_pitch, t)
 	_roll = lerp_angle_deg(_roll, _target_roll, t)
 	queue_redraw()
 
 
-static func lerp_angle_deg(from: float, to: float, weight: float) -> float:
-	return rad_to_deg(lerp_angle(deg_to_rad(from), deg_to_rad(to), weight))
-
-
 func _draw() -> void:
 	var center := size / 2.0
 	var radius := minf(size.x, size.y) / 2.0 - 10.0