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/alias_preflight_box.hako

@@ -0,0 +1,28 @@
+// alias_preflight_box.hako — AliasPreflightBox
+// Responsibility: Early check for using-alias heads in Stage‑1 names.
+// Input: raw dotted label (e.g., "alias.something.method"), UsingResolverBox instance.
+// Behavior: If dotted, verify alias head is known; otherwise print a stable one-line error and return 0.
+// Non‑Responsibility: Namespace normalization or emit.
+
+using "lang/src/compiler/pipeline_v2/using_resolver_box.hako" as UsingResolverBox
+using "lang/src/compiler/pipeline_v2/regex_flow.hako" as RegexFlow
+
+static box AliasPreflightBox {
+  // Returns 1 if OK, 0 if alias head is unresolved (prints error).
+  check_head(raw_label, r_state) {
+    if raw_label == null { return 1 }
+    if r_state == null { return 1 }
+    local s = "" + raw_label
+    local dot = RegexFlow.find_from(s, ".", 0)
+    if dot < 0 { return 1 }
+    local head = s.substring(0, dot)
+    if head == null || head == "" { return 1 }
+    if UsingResolverBox.resolve_namespace_alias(r_state, head) == null {
+      print("[ERROR] Unresolved using alias: " + head)
+      return 0
+    }
+    return 1
+  }
+}
+
+static box AliasPreflightMain { main(args) { return 0 } }