feat(progression): XP, niveaux et popups flottants (+N XP)

Nouvel autoload PlayerProgress qui gagne de l'XP sur:
- cassage de blocs (gain variable selon le type: perle, épave, coral > sand)
- collecte de perles (+25)
- crafting (+10)
- paliers de profondeur atteints (+20 à 10/25/50/75/100m)

HUD: barre XP + label niveau en bas centre, bannière "NIVEAU X" au level-up
(son bulle + fade). Popup 3D "+N XP" spawn au point d'action.

Boucle court-terme dopamine: chaque action = feedback visuel immédiat.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/progression/XpPopup.gd

@@ -0,0 +1,42 @@
+extends Node3D
+
+var _label: Label3D = null
+var _elapsed: float = 0.0
+var _lifetime: float = 1.4
+var _drift: float = 1.6
+
+
+static func spawn(parent: Node, text: String, world_pos: Vector3, color: Color = Color(1.0, 0.9, 0.2)) -> void:
+	var popup := Node3D.new()
+	popup.set_script(load("res://scripts/progression/XpPopup.gd"))
+	parent.add_child(popup)
+	popup.global_position = world_pos
+	popup.configure(text, color)
+
+
+func configure(text: String, color: Color) -> void:
+	_label = Label3D.new()
+	_label.text = text
+	_label.font_size = 54
+	_label.outline_size = 6
+	_label.modulate = color
+	_label.billboard = BaseMaterial3D.BILLBOARD_ENABLED
+	_label.no_depth_test = true
+	_label.render_priority = 4
+	_label.pixel_size = 0.008
+	add_child(_label)
+
+
+func _process(delta: float) -> void:
+	_elapsed += delta
+	var t: float = _elapsed / _lifetime
+	position.y += _drift * delta
+	if _label != null:
+		var alpha: float = 1.0 - clampf(t, 0.0, 1.0)
+		var c: Color = _label.modulate
+		c.a = alpha
+		_label.modulate = c
+		var scale_factor: float = 1.0 + (1.0 - alpha) * 0.4
+		_label.scale = Vector3.ONE * scale_factor
+	if _elapsed >= _lifetime:
+		queue_free()