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!(

src/parser/expr_cursor.rs

@@ -246,6 +246,19 @@ impl ExprParserWithCursor {
                 continue;
             }
 
+            // 添字アクセス target[index]
+            if cursor.match_token(&TokenType::LBRACK) {
+                cursor.advance(); // consume '['
+                let index_expr = Self::parse_expression(cursor)?;
+                cursor.consume(TokenType::RBRACK)?;
+                expr = ASTNode::Index {
+                    target: Box::new(expr),
+                    index: Box::new(index_expr),
+                    span: Span::unknown(),
+                };
+                continue;
+            }
+
             break;
         }
 

src/parser/mod.rs

@@ -276,7 +276,9 @@ impl NyashParser {
 
             // 左辺が代入可能な形式かチェック
             match &expr {
-                ASTNode::Variable { .. } | ASTNode::FieldAccess { .. } => Ok(ASTNode::Assignment {
+                ASTNode::Variable { .. }
+                | ASTNode::FieldAccess { .. }
+                | ASTNode::Index { .. } => Ok(ASTNode::Assignment {
                     target: Box::new(expr),
                     value,
                     span: Span::unknown(),