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/flow_entry.hako

@@ -0,0 +1,26 @@
+// flow_entry.hako — Pipeline v2 entry box（emit-only）
+// Guard: This box performs no execution. Returns MIR(JSON) as text.
+
+using "lang/src/compiler/pipeline_v2/pipeline.hako" as PipelineV2
+
+static box FlowEntryBox {
+  // Emit legacy v0 JSON（call/boxcall/newbox）。最小入力: Stage‑1 JSON 文字列
+  emit_v0_from_ast(ast_json, prefer_cfg) {
+    return PipelineV2.lower_stage1_to_mir(ast_json, prefer_cfg)
+  }
+  // Emit v0 with using context (alias/module maps) — prefer this when names need resolution
+  emit_v0_from_ast_with_usings(ast_json, prefer_cfg, usings_json, modules_json) {
+    print("[DEBUG FlowEntry] emit_v0_from_ast_with_usings called")
+    local result = PipelineV2.lower_stage1_to_mir_with_usings(ast_json, prefer_cfg, usings_json, modules_json)
+    print("[DEBUG FlowEntry] result=" + result)
+    return result
+  }
+
+  // Emit v1 → v0 互換 JSON（unified mir_call を一旦生成して適応）。自己ホスト実行向け
+  emit_v1_compat_from_ast(ast_json, prefer_cfg) {
+    return PipelineV2.lower_stage1_to_mir_v1_compat(ast_json, prefer_cfg)
+  }
+
+  // No-op entry（箱ガード用）
+  main(args) { return 0 }
+}