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/map_helpers_box.hako

@@ -0,0 +1,43 @@
+// map_helpers_box.hako — Pipeline用の軽量ヘルパ（MapBoxの型付き取得）
+
+using "lang/src/shared/common/string_helpers.hako" as StringHelpers
+using "lang/src/shared/common/box_helpers.hako" as BoxHelpers
+static box MapHelpersBox {
+  _raw_get(m, key) {
+    return BoxHelpers.map_get(m, key)
+  }
+
+  _to_i64(v) { return StringHelpers.to_i64(v) }
+
+  get_str(m, key) {
+    local v = me._raw_get(m, key)
+    if v == null { return "" }
+    if BoxHelpers.is_map(v) == 1 {
+      local inner = BoxHelpers.map_get(v, "value")
+      if inner != null { return "" + inner }
+    }
+    return "" + v
+  }
+
+  get_i64(m, key) {
+    local v = me._raw_get(m, key)
+    return BoxHelpers.value_i64(v)
+  }
+
+  expect_str(m, key) { return me.get_str(m, key) }
+  expect_i64(m, key) { return me.get_i64(m, key) }
+
+  opt_str(m, key, def) {
+    local s = me.get_str(m, key)
+    if s == null || s == "" { return def }
+    return s
+  }
+
+  opt_i64(m, key, def) {
+    local n = me.get_i64(m, key)
+    if n == null { return def }
+    return n
+  }
+}
+
+static box MapHelpersStub { main(args) { return 0 } }