Space Lua: Add load function (#1631)

* Add `load` Lua builtin

Signed-off-by: Matouš Jan Fialka <mjf@mjf.cz>

client/space_lua/stdlib/load_test.lua

@@ -0,0 +1,74 @@
+local function assert_eq(actual, expected, msg)
+  if actual ~= expected then
+    error('assert_eq failed: ' .. msg)
+  end
+end
+
+local function assert_true(cond, msg)
+  if not cond then
+    error('assert_true failed: ' .. msg)
+  end
+end
+
+local function assert_false(cond, msg)
+  if cond then
+    error('assert_false failed: ' .. msg)
+  end
+end
+
+-- 1. Successful compile -> returns function; call yields value
+
+do
+  local f, err = load("return 40 + 2")
+
+  assert_eq(type(f), "function", "load must return function on success")
+  assert_eq(err, nil, "load must not return error on success")
+  assert_eq(f(), 42, "loaded chunk executes and returns")
+end
+
+-- 2. No explicit return -> nil on call
+
+do
+  local f, err = load("local x = 1; x = x + 1")
+
+  assert_eq(type(f), "function", "load returns function even without returns")
+  assert_eq(err, nil, "no error expected")
+  assert_eq(f(), nil, "chunk without return yields nil")
+end
+
+-- 3. Syntax error -> (nil, "error message"), no throw on load
+
+do
+  local f, err = load("return 1 +")
+
+  assert_eq(f, nil, "syntax error: first result is nil")
+  assert_eq(type(err), "string", "syntax error: second result is error string")
+end
+
+-- 4. Runtime error when calling the returned function
+
+do
+  local f = load("return 1 + {}") -- arithmetic on non-number at runtime
+  local ok, res = pcall(f)
+
+  assert_false(ok, "pcall must be false on runtime error")
+  assert_eq(type(res), "string", "pcall error message is string")
+end
+
+-- 5. Global reads are visible in loaded chunk
+
+do
+  g = 41
+  local f = load("return g + 1")
+
+  assert_eq(f(), 42, "loaded chunk sees global env (read)")
+end
+
+-- 6. Global writes from loaded chunk update globals
+do
+  h = 0
+  local f = load("h = 9")
+
+  assert_eq(f(), nil, "no explicit return")
+  assert_eq(h, 9, "global updated by loaded chunk (write)")
+end