Replace godot_db_manager plugin with native DB, port lod plugin

godot_db_manager was incompatible with Godot 4 (used WindowDialog, Tabs,
PopupPanel which were all removed). Replace with a minimal Database.gd
that parses the same ahog.json format and exposes the same surface API
(get_table_by_name, get_data_at_row_idx, edit_data, save_db, etc.) used
by the M* model classes — no changes needed in MBase/MScene/MLevel/MSetting.

Also port the lod plugin: fix class_name syntax (Godot 4 uses @icon
separately from class_name extends) and Particles -> GPUParticles3D.

Rewrite Global.gd async scene loading: the convert-3to4 tool mapped
load_interactive -> load_threaded_request but those have different APIs
(stage count, poll vs. status enum). Reimplement using the new
load_threaded_get_status / load_threaded_get pair.

Clean project.godot: drop the old _global_script_classes table (Godot 4
uses inline class_name declarations), remove gddb_* autoloads, and
remove the godot_db_manager entry from editor_plugins.

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

addons/lod/lod_spot_light.gd

@@ -1,7 +1,8 @@
 # Copyright © 2020 Hugo Locurcio and contributors - MIT License
 # See `LICENSE.md` included in the source distribution for details.
+@icon("res://addons/lod/lod_spot_light.svg")
+class_name LODSpotLight
 extends SpotLight3D
-class_name LODSpotLight, "lod_spot_light.svg"
 
 # If `false`, LOD won't update anymore. This can be used for performance comparison
 # purposes.