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/emit/common/call_emit_box.hako

@@ -0,0 +1,51 @@
+// call_emit_box.hako — CallEmitBox: construct MIR(JSON v0) nodes for call-family ops
+// Responsibility: return MapBox nodes for call/boxcall/mir_call variants.
+// Notes:
+// - Args are expected as an ArrayBox of integer register ids.
+// - Callers may optionally attach  (string like "[1,2]") for legacy rebuild paths.
+
+using "lang/src/shared/common/string_helpers.hako" as StringHelpers
+
+static box CallEmitBox {
+  make_call(name, arg_ids, dst) {
+    return {op: "call", name: name, args: arg_ids, dst: dst}
+  }
+
+  make_boxcall(method, recv_id, arg_ids, dst) {
+    return {op: "boxcall", method: method, recv: recv_id, args: arg_ids, dst: dst}
+  }
+
+  // MIR v1 experimental variants
+  make_mir_call_global(name, arg_ids, dst) {
+    local callee = {type: "Global", name: name}
+    return {op: "mir_call", dst: dst, callee: callee, args: arg_ids}
+  }
+  make_mir_call_extern(name, arg_ids, dst) {
+    local callee = {type: "Extern", name: name}
+    return {op: "mir_call", dst: dst, callee: callee, args: arg_ids}
+  }
+
+  make_mir_call_method(method, recv_id, arg_ids, dst) {
+    local callee = {type: "Method", method: method, receiver: recv_id}
+    return {op: "mir_call", dst: dst, callee: callee, args: arg_ids}
+  }
+
+  make_mir_call_module(name, arg_ids, dst) {
+    local canon = me._canonical_module_name(name, arg_ids.size())
+    local callee = {type: "ModuleFunction", name: canon}
+    return {op: "mir_call", dst: dst, callee: callee, args: arg_ids}
+  }
+
+  make_mir_call_constructor(box_type, arg_ids, dst) {
+    local callee = {type: "Constructor", box_type: box_type}
+    return {op: "mir_call", dst: dst, callee: callee, args: arg_ids}
+  }
+
+  _canonical_module_name(name, arity) {
+    if name == null { return "" }
+    if name.indexOf("/") >= 0 { return name }
+    return name + "/" + StringHelpers.int_to_str(arity)
+  }
+}
+
+static box CallEmitBoxStub { main(args) { return 0 } }