restore(lang/compiler): bring back lang/src/compiler from e917d400; add Hako index canaries and docs; implement Rust-side index operator (Array/Map get/set) with Fail‑Fast diagnostics

- restore: lang/src/compiler/** (parser/emit/builder/pipeline_v2) from e917d400
- docs: docs/development/selfhosting/index-operator-hako.md
- smokes(hako): tools/smokes/v2/profiles/quick/core/index_operator_hako.sh (opt-in)
- smokes(vm): adjust index_operator_vm.sh for semicolon gate + stable error text
- rust/parser: allow IndexExpr and assignment LHS=Index; postfix parse LBRACK chain
- rust/builder: lower arr/map index to BoxCall get/set; annotate array/map literals; Fail‑Fast for unsupported types
- CURRENT_TASK: mark Rust side done; add Hako tasks checklist

Note: files disappeared likely due to branch FF to a lineage without lang/src/compiler; no explicit delete commit found. Added anchor checks and suggested CI guard in follow-up.

lang/src/compiler/pipeline_v2/method_extract_box.hako

@@ -0,0 +1,12 @@
+// MethodExtractBox — Stage‑1 JSON から Return(Method recv, method, args) を抽出（整数引数のみ; recvは無視）
+// Delegation to Stage1IntArgsExtractBox (unified implementation)
+using "lang/src/compiler/pipeline_v2/stage1_int_args_extract_box.hako" as Unified
+
+static box MethodExtractBox {
+  // Returns { method: String, args: [Int,...] } or null
+  extract_return_method_ints(ast_json) {
+    return Unified.extract_return_method_ints(ast_json)
+  }
+}
+
+static box MethodExtractStub { main(args) { return 0 } }