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/debug/debug_box.hako

@@ -0,0 +1,39 @@
+// DebugBox — conditional debug output aggregator (migrated to lang/src/compiler)
+box DebugBox {
+  enabled
+  birth() {
+    me.enabled = 0
+    return 0
+  }
+  set_enabled(v) {
+    me.enabled = v
+    return 0
+  }
+  log(msg) {
+    if me.enabled {
+      local c = new ConsoleBox()
+      c.println("[DEBUG] " + msg)
+    }
+    return 0
+  }
+  info(msg) {
+    if me.enabled {
+      local c = new ConsoleBox()
+      c.println("[INFO] " + msg)
+    }
+    return 0
+  }
+  error(msg) {
+    if me.enabled {
+      local c = new ConsoleBox()
+      c.println("[ERROR] " + msg)
+    }
+    return 0
+  }
+}
+
+// Include stub to satisfy current include lowering (expects a static box)
+static box DebugStub {
+  main(args) { return 0 }
+}
+