feat: more clear definition and first base implementation for lua

Co-authored-by: Copilot <copilot@github.com>

lib/bds/scripting/runtime.ex

@@ -0,0 +1,24 @@
+defmodule BDS.Scripting.Runtime do
+  @moduledoc """
+  Behaviour for user-script runtimes hosted by bDS.
+
+  The runtime boundary is intentionally narrow: syntax validation and
+  bounded entrypoint execution.
+  """
+
+  @type source :: String.t()
+  @type entrypoint :: String.t()
+  @type args :: [term()]
+  @type progress_event :: map()
+  @type progress_callback :: (progress_event() -> any())
+  @type execution_option ::
+          {:timeout, non_neg_integer() | :infinity}
+          | {:max_reductions, pos_integer() | :none}
+          | {:spawn_opts, [term()]}
+          | {:on_progress, progress_callback()}
+          | {:capabilities, map()}
+
+  @callback validate(source()) :: :ok | {:error, term()}
+  @callback execute(source(), entrypoint(), args(), [execution_option()]) ::
+              {:ok, term()} | {:error, term()}
+end