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_gauge.gd

@@ -1,5 +1,5 @@
 class_name HudGauge
-extends Control
+extends HudInstrument
 
 # Generic bar gauge (vertical or horizontal) with a short label and numeric
 # readout — used for speed, altitude, and thrust. A dumb display fed by
@@ -9,8 +9,6 @@ const BG_COLOR := Color(0.05, 0.07, 0.11, 0.55)
 const FRAME_COLOR := Color(0.5, 0.85, 1.0, 0.7)
 const TEXT_COLOR := Color(0.85, 0.92, 1.0, 0.9)
 
-const SMOOTHING := 12.0
-
 @export var label := "SPD"
 @export var max_value := 100.0
 @export var horizontal := false
@@ -27,7 +25,7 @@ func set_value(value: float) -> void:
 
 
 func _process(delta: float) -> void:
-	var t := 1.0 - exp(-SMOOTHING * delta)  # frame-rate independent
+	var t := _smoothing_weight(delta)
 	_value = lerpf(_value, _target, t)
 	queue_redraw()