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/ast/utils.rs

@@ -26,6 +26,7 @@ impl ASTNode {
             ASTNode::BinaryOp { .. } => "BinaryOp",
             ASTNode::MethodCall { .. } => "MethodCall",
             ASTNode::FieldAccess { .. } => "FieldAccess",
+            ASTNode::Index { .. } => "Index",
             ASTNode::New { .. } => "New",
             ASTNode::This { .. } => "This",
             ASTNode::Me { .. } => "Me",
@@ -78,6 +79,7 @@ impl ASTNode {
             ASTNode::FromCall { .. } => ASTNodeType::Expression,
             ASTNode::ThisField { .. } => ASTNodeType::Expression,
             ASTNode::MeField { .. } => ASTNodeType::Expression,
+            ASTNode::Index { .. } => ASTNodeType::Expression,
             ASTNode::MatchExpr { .. } => ASTNodeType::Expression,
             ASTNode::QMarkPropagate { .. } => ASTNodeType::Expression,
             ASTNode::Lambda { .. } => ASTNodeType::Expression,
@@ -312,6 +314,9 @@ impl ASTNode {
             ASTNode::MapLiteral { entries, .. } => {
                 format!("MapLiteral({} entries)", entries.len())
             }
+            ASTNode::Index { target, index, .. } => {
+                format!("Index(target={:?}, index={:?})", target, index)
+            }
             ASTNode::ScopeBox { .. } => "ScopeBox".to_string(),
         }
     }
@@ -342,6 +347,7 @@ impl ASTNode {
             ASTNode::BinaryOp { span, .. } => *span,
             ASTNode::MethodCall { span, .. } => *span,
             ASTNode::FieldAccess { span, .. } => *span,
+            ASTNode::Index { span, .. } => *span,
             ASTNode::New { span, .. } => *span,
             ASTNode::This { span, .. } => *span,
             ASTNode::Me { span, .. } => *span,