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/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(),