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.

src/parser/expr/call.rs

@@ -192,6 +192,16 @@ impl NyashParser {
                         span: Span::unknown(),
                     };
                 }
+            } else if self.match_token(&TokenType::LBRACK) {
+                self.advance(); // consume '['
+                must_advance!(self, _unused, "index expression parsing");
+                let index_expr = self.parse_expression()?;
+                self.consume(TokenType::RBRACK)?;
+                expr = ASTNode::Index {
+                    target: Box::new(expr),
+                    index: Box::new(index_expr),
+                    span: Span::unknown(),
+                };
             } else if self.match_token(&TokenType::QUESTION) {
                 let nt = self.peek_token();
                 let is_ender = matches!(