feat: more clear definition and first base implementation for lua

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

test/bds/scripting/lua_test.exs

@@ -0,0 +1,43 @@
+defmodule BDS.Scripting.LuaTest do
+  use ExUnit.Case, async: true
+
+  test "validates Lua source" do
+    assert :ok = BDS.Scripting.validate("function main() return 42 end")
+  end
+
+  test "rejects invalid Lua source" do
+    assert {:error, {:compile_error, _details}} = BDS.Scripting.validate("function main(")
+  end
+
+  test "executes the configured entrypoint in a sandboxed Lua runtime" do
+    source = "function main(a, b) return a + b end"
+
+    assert {:ok, 42} = BDS.Scripting.execute(source, "main", [19, 23])
+  end
+
+  test "exposes progress reporting through the host boundary" do
+    parent = self()
+
+    source = """
+    function main()
+      bds.report_progress({phase = 'fetch', current = 1, total = 2})
+      bds.report_progress({phase = 'write', current = 2, total = 2})
+      return 'done'
+    end
+    """
+
+    callback = fn progress -> send(parent, {:progress, progress}) end
+
+    assert {:ok, "done"} = BDS.Scripting.execute(source, "main", [], on_progress: callback)
+
+    assert_receive {:progress, %{"phase" => "fetch", "current" => 1, "total" => 2}}
+    assert_receive {:progress, %{"phase" => "write", "current" => 2, "total" => 2}}
+  end
+
+  test "enforces reduction limits" do
+    source = "function main() while true do end end"
+
+    assert {:error, {:reductions_exceeded, _count}} =
+             BDS.Scripting.execute(source, "main", [], timeout: 1_000, max_reductions: 100)
+  end
+end