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_instrument.gd

@@ -0,0 +1,17 @@
+class_name HudInstrument
+extends Control
+
+# Shared base for the HUD's procedural instruments (heading tape, attitude
+# indicator, gauges): the exponential-smoothing weight calc and the
+# angle-aware lerp helper they all use. Each instrument still owns its own
+# _process (smoothing 1-2 distinct fields) and _draw (entirely bespoke).
+
+const SMOOTHING := 12.0
+
+
+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 _smoothing_weight(delta: float) -> float:
+	return 1.0 - exp(-SMOOTHING * delta)  # frame-rate independent