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/execution_pipeline_box.hako

@@ -0,0 +1,49 @@
+// ExecutionPipelineBox — Orchestrate Parser → Emit (emit-only; no execution)
+using lang.compiler.parser.box as ParserBoxMod
+// Parser dependencies (Phase 2 refactoring: hierarchical structure)
+using lang.compiler.parser.scan.parser_string_utils_box
+using lang.compiler.parser.scan.parser_ident_scan_box
+using lang.compiler.parser.scan.parser_number_scan_box
+using lang.compiler.parser.scan.parser_string_scan_box
+using lang.compiler.parser.using.using_collector_box
+using lang.compiler.parser.expr.parser_expr_box
+using lang.compiler.parser.expr.parser_peek_box
+using lang.compiler.parser.expr.parser_literal_box
+using lang.compiler.parser.stmt.parser_stmt_box
+using lang.compiler.parser.stmt.parser_control_box
+using lang.compiler.parser.stmt.parser_exception_box
+using lang.compiler.stage1.json_program_box
+using lang.compiler.stage1.emitter_box as EmitterBoxMod
+using "lang/src/compiler/pipeline_v2/backend_box.hako" as BackendBoxMod
+
+box ExecutionPipelineBox {
+  backend_name
+  backend
+
+  birth(name) {
+    // Optional backend tag (no execution here)
+    if name == null { name = "vm" }
+    me.backend_name = name
+    me.backend = new BackendBox(name)
+    return 0
+  }
+
+  // Run with source text; stage3_flag=1 enables Stage‑3 acceptance in parser
+  run_source(src, stage3_flag) {
+    if src == null { src = "return 0" }
+    if stage3_flag == null { stage3_flag = 0 }
+    // Parse
+    local p = new ParserBox()
+    if stage3_flag == 1 { p.stage3_enable(1) }
+    p.extract_usings(src)
+    local usings = p.get_usings_json()
+    local ast = p.parse_program2(src)
+    // Emit Stage‑1 JSON with meta.usings
+    local json = EmitterBox.emit_program(ast, usings)
+    if json == null || json.size() == 0 { return 1 }
+    print(json)
+    return 0
+  }
+}
+
+static box ExecutionPipelineStub { main(args) { return 0 } }