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/erase_tool.gd

@@ -0,0 +1,41 @@
+class_name EraseTool
+extends EditorTool
+## Deletes an object, or clears a tile back to id 0 (the reserved empty/air
+## id -- world.md §3.2 / TileDef doc comment).
+##
+## `args`: either `{"object_id": ...}` to delete an object, or
+## `{"layer", "x", "y", "z"}` to clear a tile (delegates to BrushTool with
+## `tile_id = 0` so both paths share one undo/redo implementation).
+
+
+func _init() -> void:
+	tool_id = "erase"
+
+
+func apply(core: EditorCore, args: Dictionary) -> Variant:
+	if args.has("object_id"):
+		return _erase_object(core, args["object_id"])
+	return _erase_tile(core, args)
+
+
+func _erase_object(core: EditorCore, object_id: String) -> bool:
+	var idx: int = core.model.get_object_index(object_id)
+	if idx == -1:
+		return false
+	var obj: ChunkDataDecoder.WorldObjectData = core.model.payload.objects[idx]
+
+	core.undo_redo.begin("Delete '%s'" % object_id)
+	core.undo_redo.add_do(core.model.remove_object_raw.bind(object_id))
+	core.undo_redo.add_undo(core.model.insert_object_raw.bind(obj, idx))
+	core.undo_redo.commit()
+
+	if core.selection.get_selected() == object_id:
+		core.selection.clear()
+	return true
+
+
+func _erase_tile(core: EditorCore, args: Dictionary) -> bool:
+	var brush: EditorTool = core.tools.get_tool("brush")
+	var tile_args: Dictionary = args.duplicate()
+	tile_args["tile_id"] = 0
+	return bool(brush.apply(core, tile_args))