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/pipeline_emit_box.hako

@@ -0,0 +1,15 @@
+// pipeline_emit_box.hako — PipelineEmitBox
+// Responsibility: small wrappers that emit MIR(JSON v0) and apply LocalSSA ensures
+
+using "lang/src/compiler/pipeline_v2/emit_call_box.hako" as EmitCallBox
+using "lang/src/compiler/pipeline_v2/local_ssa_box.hako" as LocalSSABox
+
+static box PipelineEmitBox {
+  // Emit Call(name, int-args) → JSON v0, wrapped with LocalSSA ensures
+  emit_call_int_args_v0(name, args) {
+    local j = EmitCallBox.emit_call_int_args(name, args)
+    return LocalSSABox.ensure_calls(LocalSSABox.ensure_cond(j))
+  }
+}
+
+static box PipelineEmitMain { main(args) { return 0 } }