Add developers/.gdignore; let Godot 4 canonicalise saved scenes

The developers/aurelien sandbox scenes still reference legacy non-LOD
mesh paths (sm_book.mesh, sm_candlestick.mesh, ...) that don't exist
anymore — the production scenes were migrated to *_lod0/1/2.mesh.
Godot's full-project file scanner reads every .tscn at editor open
and emits 'Cannot open file' errors for each broken ext_resource,
even for scenes that the game never loads.

Mark developers/ as out-of-scope for Godot's filesystem walker with
a .gdignore at its root. The .import / .uid sidecars Godot had
generated for files inside (CheckLightmap.exr.import,
CheckLightmap.gd.uid) are auto-removed by the editor since they are
now orphaned metadata — only the source files (.tscn, .gd, .exr,
.lmbake) remain. Re-enabling the sandbox later just means deleting
the .gdignore; Godot will regenerate the sidecars.

The other diffs in this commit are Godot 4 canonicalising whatever
.tscn / .tres files were touched in the editor session that ran the
LightmapGI bake attempt: ext_resource uid= attributes added, format
bumped 2 -> 3, property order normalised. No behavioural change.

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

assets/props/stool b/sm_stool_2.tscn

@@ -1,10 +1,8 @@
-[gd_scene load_steps=2 format=2]
+[gd_scene format=3 uid="uid://8v3e4xmvehhx"]
 
-[ext_resource path="res://assets/props/stool b/sm_stool_2_lod1.mesh" type="ArrayMesh" id=1]
+[ext_resource type="ArrayMesh" path="res://assets/props/stool b/sm_stool_2_lod1.mesh" id="1"]
 
-[node name="sm_stool_b" type="Node3D"]
+[node name="sm_stool_b" type="Node3D" unique_id=782579931]
 
-[node name="sm_stool_2_lod1" type="MeshInstance3D" parent="."]
-use_in_baked_light = true
-mesh = ExtResource( 1 )
-surface_material_override/0 = null
+[node name="sm_stool_2_lod1" type="MeshInstance3D" parent="." unique_id=873563256]
+mesh = ExtResource("1")