feat(module): add module loader and import resolution

src/main/kotlin/lumen/module/ModuleLoader.kt

@@ -0,0 +1,60 @@
+package lumen.module
+
+import lumen.LumenError
+import lumen.RuntimeError
+import lumen.ast.ImportStmt
+import lumen.ast.Stmt
+import lumen.compiler.Compiler
+import lumen.lexer.Lexer
+import lumen.parser.Parser
+import lumen.runtime.NilValue
+import lumen.runtime.Value
+import lumen.vm.Vm
+import java.io.File
+
+class ModuleError(val path: String, val source: String, val error: LumenError) :
+    RuntimeException(error.message)
+
+class ModuleLoader(
+    private val vm: Vm,
+    private val globalMutability: MutableMap<String, Boolean> = mutableMapOf()
+) {
+    private val loaded = HashSet<String>()
+    private val loading = HashSet<String>()
+
+    fun load(file: File): Value {
+        val key = file.canonicalPath
+        if (key in loaded || key in loading) return NilValue
+        loading.add(key)
+        val source = file.readText()
+        try {
+            val program = Parser(Lexer(source).scan()).parseProgram()
+            loadImports(file, program)
+            val script = Compiler(globalMutability).compile(program)
+            val result = vm.interpret(script)
+            loading.remove(key)
+            loaded.add(key)
+            return result
+        } catch (error: LumenError) {
+            throw ModuleError(file.path, source, error)
+        }
+    }
+
+    fun loadImports(origin: File, program: List<Stmt>) {
+        for (stmt in program) {
+            if (stmt !is ImportStmt) continue
+            val target = resolve(origin, stmt.path)
+            if (!target.exists()) {
+                throw RuntimeError("cannot import '${stmt.path}': file not found", stmt.line, stmt.column)
+            }
+            load(target)
+        }
+    }
+
+    private fun resolve(origin: File, path: String): File {
+        val direct = File(path)
+        if (direct.isAbsolute) return direct
+        val base = origin.absoluteFile.parentFile ?: File(".")
+        return File(base, path)
+    }
+}