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/tools/object_place_tool.gd

@@ -0,0 +1,50 @@
+class_name ObjectPlaceTool
+extends EditorTool
+## Places a new catalog object into the chunk at a given transform.
+##
+## `args`: `catalog_id` (String, required), `position` (Vector3), `rotation`
+## (Vector4 quaternion, optional, default identity), `scale` (Vector3,
+## optional, default ONE), `properties` (Dictionary, optional, merged in).
+##
+## Returns the new object's id (String) on success, or "" on failure
+## (unknown catalog_id).
+
+
+func _init() -> void:
+	tool_id = "object_place"
+
+
+func apply(core: EditorCore, args: Dictionary) -> Variant:
+	var catalog_id: String = args.get("catalog_id", "")
+	if catalog_id == "":
+		push_error("ObjectPlaceTool: missing catalog_id")
+		return ""
+
+	var entry: Dictionary = {}
+	if core.catalog != null:
+		entry = core.catalog.resolve(catalog_id)
+		if entry.is_empty():
+			push_error("ObjectPlaceTool: unknown catalog_id '%s'" % catalog_id)
+			return ""
+
+	var obj := ChunkDataDecoder.WorldObjectData.new()
+	obj.id = core.model.generate_object_id()
+	obj.type = entry.get("type", "prop")
+	obj.asset_id = entry.get("asset_id", catalog_id)
+	obj.position = args.get("position", Vector3.ZERO)
+	obj.rotation = args.get("rotation", Vector4(0, 0, 0, 1))
+	obj.scale = args.get("scale", Vector3.ONE)
+
+	var props: Dictionary = (args.get("properties", {}) as Dictionary).duplicate()
+	if entry.has("display_name"):
+		props["display_name"] = entry["display_name"]
+	props["catalog_id"] = catalog_id
+	obj.properties = props
+
+	core.undo_redo.begin("Place '%s' (%s)" % [obj.id, catalog_id])
+	core.undo_redo.add_do(core.model.add_object_raw.bind(obj))
+	core.undo_redo.add_undo(core.model.remove_object_raw.bind(obj.id))
+	core.undo_redo.commit()
+
+	core.selection.select(obj.id)
+	return obj.id