Add in-game editor MVP: EditorCore + 2D tile/object editing

Implements the in-game editor's shared, data-native core and a
Control-based UI on top of it, per docs/architecture/editor.md §7.1/§7.2,
scoped to 2D per §8's own risk mitigation (the default world is flat,
CHUNK_SIZE_Z=1 -- no 3D gizmos here).

EditorCore (scripts/editor/):
- ChunkEditorModel wraps a ChunkDataDecoder.ChunkPayload directly (no
  parallel data model) and adds a TileLayerData.set_tile() mutator
  alongside the existing get_tile().
- EditorUndoRedoStack wraps Godot's built-in UndoRedo (game-usable, not
  the editor-only EditorUndoRedoManager) so every mutation is a real,
  revertible do/undo Callable pair over ChunkEditorModel's raw setters.
- SelectionManager tracks the selected object id as UI state, deliberately
  outside the undo/redo stack.
- ToolRegistry + 5 tools (BrushTool, ObjectPlaceTool, SelectTool,
  EraseTool, TransformTool -- the 5th covers move/rotate/scale, which the
  doc's tool list doesn't literally name but the task requires) is the
  single choke point every mutation goes through.
- EditorObjectCatalog (assets/editor/object_catalog.json) is a new,
  small "placeable object" catalog -- AvatarCatalog and SpawnCatalog were
  checked first and are both scoped to other concerns, so this mirrors
  their loading convention rather than reusing either.

InGameEditor UI (scripts/editor/ui/): EditorViewport (2D top-down tile
grid + object dots), EditorToolbar, PalettePanel, InspectorPanel, wired
together in InGameEditor. Built from Control nodes in code (matching
game.gd's existing dynamic-UI convention) rather than a hand-authored
.tscn.

Entry point: game.gd gates build mode behind a new toggle_build_mode
input action (F2), freezing the player and opening a fresh empty
ChunkEditorModel. Loading a *real* chunk from the backend for editing,
and saving edits back, are explicitly out of scope -- a sibling task
owns the batch-PUT/conflict-handling save path
(world_persistence_impl.md).

Verification: scripts/smoke_test_editor_core.gd decodes a real backend
fixture (backend/tests/fixtures/chunks/simple_tile_chunk.bin), drives
place/move/rotate/scale/delete/paint/erase entirely through
EditorCore.use_tool()/ToolRegistry (never touching ChunkEditorModel's
raw setters directly), undoes and redoes every step, and round-trips the
result through encode_binary()/decode_binary() to prove the edited model
stays a valid, byte-compatible ChunkPayload. 69/69 assertions pass. Full
existing Godot smoke suite (16 other scripts/smoke_test_*.gd, backend +
dedicated-server-dependent ones included) still passes with no
regressions.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y2FESCaMXeJpC9gSctiEeD

scripts/editor/object_catalog.gd

@@ -0,0 +1,68 @@
+class_name EditorObjectCatalog
+extends RefCounted
+## Catalog of placeable world objects for the in-game editor's
+## `PalettePanel` (docs/architecture/editor.md §7.2).
+##
+## Mirrors the loading convention already used twice in this codebase --
+## `scripts/avatar/avatar_catalog.gd` (AvatarCatalog) and
+## `scripts/scripting/spawn_catalog.gd` (SpawnCatalog): a data-driven JSON
+## manifest, loaded once, looked up by id. Checked both before adding this:
+## `AvatarCatalog` is avatar-attachment-specific (base_body/slot/mount_bone)
+## and `SpawnCatalog` is scoped to `ScriptBehavior`'s `spawn_from_catalog`
+## verb specifically (its own doc comment says as much) -- neither is a
+## general "placeable world object" catalog, so this is a new, small one
+## rather than a reuse of either.
+
+## Catalog manifest structure:
+## { "objects": { "id": { "type", "asset_id", "display_name", ... } } }
+var _catalog: Dictionary = {}
+var _loaded: bool = false
+var _catalog_path: String = "res://assets/editor/object_catalog.json"
+
+
+func load_catalog(path: String = _catalog_path) -> Error:
+	_catalog_path = path
+	_catalog = {}
+
+	var file := FileAccess.open(path, FileAccess.READ)
+	if file == null:
+		return FileAccess.get_open_error()
+
+	var json_text: String = file.get_as_text()
+	file.close()
+
+	var result: Variant = JSON.parse_string(json_text)
+	if result == null or not (result is Dictionary) or not (result as Dictionary).has("objects"):
+		return ERR_INVALID_DATA
+
+	_catalog = (result as Dictionary)["objects"]
+	_loaded = true
+	return OK
+
+
+func is_loaded() -> bool:
+	return _loaded
+
+
+## Resolve a catalog id to its template Dictionary ({} if unknown/unloaded).
+func resolve(id: String) -> Dictionary:
+	if not _loaded:
+		return {}
+	return _catalog.get(id, {})
+
+
+func has_object(id: String) -> bool:
+	return _loaded and _catalog.has(id)
+
+
+func get_all_ids() -> Array[String]:
+	if not _loaded:
+		return []
+	var ids: Array[String] = []
+	for id in _catalog.keys():
+		ids.append(id)
+	return ids
+
+
+func get_catalog_path() -> String:
+	return _catalog_path