restore(lang): full lang tree from ff3ef452 (306 files) — compiler, vm, shared, runner, c-abi, etc.\n\n- Restores lang/ directory (files≈306, dirs≈64) as per historical branch with selfhost sources\n- Keeps our recent parser index changes in compiler/* (merged clean by checkout)\n- Unblocks selfhost development and documentation references

lang/src/compiler/pipeline_v2/flow_entry.hako

@@ -2,6 +2,7 @@
 // Guard: This box performs no execution. Returns MIR(JSON) as text.
 
 using "lang/src/compiler/pipeline_v2/pipeline.hako" as PipelineV2
+using "lang/src/shared/json/mir_v1_adapter.hako" as MirJsonV1Adapter
 
 static box FlowEntryBox {
   // Emit legacy v0 JSON（call/boxcall/newbox）。最小入力: Stage‑1 JSON 文字列
@@ -21,6 +22,18 @@ static box FlowEntryBox {
     return PipelineV2.lower_stage1_to_mir_v1_compat(ast_json, prefer_cfg)
   }
 
+  // Emit v1 JSON with metadata.extern_c（externs を v1 の metadata に反映）
+  // externs_json: JSON array text, e.g. [{"func":"Name/Arity","symbol":"c_symbol"}]
+  emit_v1_from_ast_with_meta(ast_json, prefer_cfg, externs_json) {
+    return PipelineV2.lower_stage1_to_mir_v1_with_meta(ast_json, prefer_cfg, externs_json)
+  }
+
+  // Emit v1 JSON + metadata.extern_c を注入し、v0互換に変換
+  emit_v1_compat_from_ast_with_meta(ast_json, prefer_cfg, externs_json) {
+    local j1 = PipelineV2.lower_stage1_to_mir_v1_with_meta(ast_json, prefer_cfg, externs_json)
+    return MirJsonV1Adapter.to_v0(j1)
+  }
+
   // No-op entry（箱ガード用）
   main(args) { return 0 }
 }