Refactor bound node types for clarity

Introduce generic `CompilerPhase` trait to unify different stages of the
bound AST. Rename `LocalSlot` to `VirtualId` and introduce `StackOffset`
for clearer distinction between compile-time and run-time addressing.
Update type aliases and implementations to reflect these changes.

src/ast/compiler/lambda_collector.rs

@@ -1,21 +1,21 @@
-use crate::ast::compiler::bound_nodes::{Address, BoundKind, GlobalIdx, Node};
+use crate::ast::compiler::bound_nodes::{Address, BoundKind, CompilerPhase, GlobalIdx, Node};
 use std::collections::HashMap;
 use std::rc::Rc;
 
 /// A pass that collects all global function definitions (lambdas) into a registry.
 /// This allows the Specializer to retrieve the original AST of a function for monomorphization.
-pub struct LambdaCollector<'a, T> {
-    registry: &'a mut HashMap<GlobalIdx, Rc<Node<T>>>,
+pub struct LambdaCollector<'a, P: CompilerPhase> {
+    registry: &'a mut HashMap<GlobalIdx, Rc<Node<P>>>,
 }
 
-impl<'a, T: Clone> LambdaCollector<'a, T> {
+impl<'a, P: CompilerPhase> LambdaCollector<'a, P> {
     /// Performs a full traversal of the AST and populates the provided registry.
-    pub fn collect(node: &Node<T>, registry: &'a mut HashMap<GlobalIdx, Rc<Node<T>>>) {
+    pub fn collect(node: &Node<P>, registry: &'a mut HashMap<GlobalIdx, Rc<Node<P>>>) {
         let mut collector = Self { registry };
         collector.visit(node);
     }
 
-    fn visit(&mut self, node: &Node<T>) {
+    fn visit(&mut self, node: &Node<P>) {
         match &node.kind {
             BoundKind::Block { exprs } => {
                 for expr in exprs {