fix(mir-builder): static method arity mismatch根治 - Phase 25.x

**問題**:
- ParserStmtBox.parse_using/4 に5引数が渡される
- me.method呼び出しで instance/static 判別なし
- static method に誤って receiver 追加

**修正**:
- MeCallPolicyBox: params[0]の型で instance/static 判別
- Instance method: receiver 追加
- Static method: receiver なし
- Arity検証（NYASH_ME_CALL_ARITY_STRICT=1）

**ドキュメント**:
- docs/reference/environment-variables.md 新規作成
- docs/development/architecture/mir-logs-observability.md 更新

**テスト**:
- src/tests/mir_stage1_cli_emit_program_min.rs 追加
- 既存 stage1 テスト全てパス

Phase: 25.x

lang/src/runner/stage1_cli.hako

@@ -147,15 +147,38 @@ static box Stage1Cli {
     // and create a fresh ArrayBox from environment variables instead
     local args_safe = new ArrayBox()
 
-    // Config box: Parse all env vars into structured config
-    local cfg = Stage1CliConfigBox.from_env()
-    local mode = cfg.get("mode")
-    local debug = cfg.get("debug")
+    // 現状: MIR 型レジストリの制約により MapBox ベースの ConfigBox は本線では使用しない。
+    // env から直接読み取り、Stage1CliConfigBox は将来の構造化用として温存する。
+
+    // mode 判定（env-only）
+    local mode = "disabled"
+    if env.get("STAGE1_EMIT_PROGRAM_JSON") == "1" {
+      mode = "emit_program_json"
+    } else if env.get("STAGE1_EMIT_MIR_JSON") == "1" {
+      mode = "emit_mir_json"
+    } else if env.get("NYASH_USE_STAGE1_CLI") == "1" {
+      mode = "run"
+    }
+
+    // backend と入力まわり
+    local backend = env.get("STAGE1_BACKEND")
+    if backend == null { backend = "vm" }
+    backend = "" + backend
+
+    local source = env.get("STAGE1_SOURCE")
+    local source_text = env.get("STAGE1_SOURCE_TEXT")
+    local prog_path = env.get("STAGE1_PROGRAM_JSON")
+    local debug = env.get("STAGE1_CLI_DEBUG")
 
     // Log entry point for debugging
     if debug == "1" {
       __mir__.log("[stage1_main] args_safe at entry", args_safe)
-      print("[stage1-cli/debug] stage1_main ENTRY: mode=" + ("" + mode) + " backend=" + ("" + cfg.get("backend")))
+      print("[stage1-cli/debug] stage1_main ENTRY: mode=" + ("" + mode) + " backend=" + backend)
+      __mir__.log("[stage1_main] env-config",
+        mode,
+        backend,
+        source,
+        prog_path)
     }
 
     // Early exit if CLI is disabled
@@ -166,21 +189,6 @@ static box Stage1Cli {
       return 97
     }
 
-    // Log config toggles before dispatch
-    if debug == "1" {
-      __mir__.log("[stage1_main] config",
-        cfg.get("mode"),
-        cfg.get("backend"),
-        cfg.get("source_path"),
-        cfg.get("program_json_path"))
-    }
-
-    // Extract config fields
-    local source = cfg.get("source_path")
-    local source_text = cfg.get("source_text")
-    local prog_path = cfg.get("program_json_path")
-    local backend = cfg.get("backend")
-
     // Dispatch by mode
     if mode == "emit_program_json" {
       if source == null || source == "" {