Add ScriptBehavior DSL: data-driven, non-Turing-complete UGC scripting

Implements the DSL that docs/architecture/scripting_sandbox.md recommends
as the primary (and, for now, sufficient) approach in place of running
untrusted GDScript in-process: a fixed set of trigger/condition/action
verbs, expressed as plain JSON data, interpreted by trusted GDScript with
zero ability to reference engine singletons, call arbitrary methods, or
execute code.

- ScriptBehavior (scripts/scripting/script_behavior.gd): parses and
  validates rules from JSON/Dictionary data. Every trigger, condition
  type/op, and verb is checked against an explicit whitelist at parse
  time; malformed or unknown entries are dropped and recorded in
  rejected_rules, never partially executed. Carries an optional
  content_flags passthrough so this data type isn't hostile to future
  moderation-queue integration (docs/architecture/content_moderation.md).
- ScriptInterpreterNode (scripts/scripting/script_interpreter.gd): the
  only code that executes rules. Triggers: on_interact (proximity +
  "interact" input action), on_enter (edge-triggered Area3D overlap),
  on_touch (continuous overlap), on_timer (physics-clocked Timer). Verbs:
  give_item, play_sound, move, rotate, spawn_from_catalog, set_timer --
  each a fixed, hand-written function; verb dispatch is an explicit
  match(), never call()-by-name from data. move/rotate animate via Tween
  rather than teleporting.
- PlayerInventory (scripts/scripting/player_inventory.gd): a small
  per-player item-count stub for give_item to target, since no real
  inventory/entity-state system exists yet (M9 Economy). Attached to
  scenes/player.tscn as "Inventory".
- SpawnCatalog (scripts/scripting/spawn_catalog.gd): trusted catalog_id ->
  asset_id lookup for spawn_from_catalog, mirroring the AvatarCatalog
  data-driven-JSON convention. Rule data only ever supplies a catalog_id
  key, never a resource path.
- Wired a real demo ("ScriptedChest") into scenes/worlds/test_level.tscn
  with an on_interact rule (give_item + play_sound + rotate), plus an
  "interact" input action (project.godot).
- scripts/smoke_test_script_behavior.gd: exercises every trigger type,
  every verb, and every condition type against the real interpreter and
  the wired demo scene, and proves unknown/malformed/unsafe rules
  (unknown verb, unknown trigger, non-scalar smuggled arguments, etc.)
  are rejected at parse time and never run -- 50/50 checks pass.

scenes/worlds/test_level.tscn

@@ -1,4 +1,6 @@
-[gd_scene load_steps=1 format=3 uid="uid://c49a36fc7db_test_level"]
+[gd_scene load_steps=2 format=3 uid="uid://c49a36fc7db_test_level"]
+
+[ext_resource type="Script" path="res://scripts/scripting/script_interpreter.gd" id="1_scripted_chest"]
 
 [sub_resource type="BoxShape3D" id="BoxShape3D_ground"]
 size = Vector3(40, 1, 40)
@@ -44,6 +46,12 @@ radius = 0.5
 subdivisions_ring = 8
 subdivisions_height = 8
 
+[sub_resource type="BoxShape3D" id="BoxShape3D_chest"]
+size = Vector3(1, 0.8, 0.6)
+
+[sub_resource type="BoxMesh" id="BoxMesh_chest"]
+size = Vector3(1, 0.8, 0.6)
+
 [sub_resource type="Environment" id="WorldEnvironment_env"]
 background_mode = 1
 background_color = Color(0.45, 0.62, 0.82, 1)
@@ -109,6 +117,18 @@ shape = SubResource("SphereShape3D_prop5")
 [node name="MeshInstance3D" type="MeshInstance3D" parent="Props/Prop5_SmallSphere"]
 mesh = SubResource("SphereMesh_prop5")
 
+[node name="ScriptedChest" type="StaticBody3D" parent="."]
+position = Vector3(0, 1.0, 1.5)
+script = ExtResource("1_scripted_chest")
+behavior_path = "res://assets/scripting/behaviors/demo_chest.json"
+interact_radius = 2.5
+
+[node name="CollisionShape3D" type="CollisionShape3D" parent="ScriptedChest"]
+shape = SubResource("BoxShape3D_chest")
+
+[node name="MeshInstance3D" type="MeshInstance3D" parent="ScriptedChest"]
+mesh = SubResource("BoxMesh_chest")
+
 [node name="Sun" type="DirectionalLight3D" parent="."]
 rotation = Vector3(-0.95, -0.5, 0)
 light_energy = 1.3