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/dumper.rs

@@ -1,5 +1,4 @@
-use crate::ast::compiler::bound_nodes::{BoundKind, Node};
-use std::fmt::Debug;
+use crate::ast::compiler::bound_nodes::{BoundKind, CompilerPhase, Node};
 
 /// Human-readable AST dumper for the bound AST.
 pub struct Dumper {
@@ -9,7 +8,7 @@ pub struct Dumper {
 
 impl Dumper {
     /// Produces a formatted string representation of the given bound AST node and its children.
-    pub fn dump<T: Debug>(node: &Node<T>) -> String {
+    pub fn dump<P: CompilerPhase>(node: &Node<P>) -> String {
         let mut dumper = Self {
             output: String::new(),
             indent: 0,
@@ -24,14 +23,14 @@ impl Dumper {
         }
     }
 
-    fn log<T: Debug>(&mut self, label: &str, node: &Node<T>) {
+    fn log<P: CompilerPhase>(&mut self, label: &str, node: &Node<P>) {
         self.write_indent();
         self.output.push_str(label);
         self.output
             .push_str(&format!(" <Metadata: {:?}>\n", node.ty));
     }
 
-    fn visit<T: Debug>(&mut self, node: &Node<T>) {
+    fn visit<P: CompilerPhase>(&mut self, node: &Node<P>) {
         match &node.kind {
             BoundKind::Nop => self.log("Nop", node),
             BoundKind::Constant(v) => {