Add example and refine local inlining

Add a new example file `examples/def_local_inlining.myc` to demonstrate
and test local inlining.

Refine the optimizer to handle local inlining more robustly by:
- Passing a `SubstitutionMap` to `visit_node` to track local variable
  substitutions.
- Enabling constant propagation for local variables by checking
  `sub.locals` in `BoundKind::Get`.
- Updating `BoundKind::DefLocal` and `BoundKind::Set` to maintain the
  substitution map for local variables.
- Adjusting `try_beta_reduce` to use the optimizer's `visit_node` with
  the substitution map for inlining.
- Re-indexing upvalues correctly when inlining captured variables into
  lambdas.

examples/def_local_inlining.myc

@@ -0,0 +1,23 @@
+;; examples/def_local_inlining.myc
+;; Demonstrates potential for DefLocal-Inlining (Phase 2.5)
+
+(fn []
+  (do
+    ;; 1. Simple constant propagation
+    (def x 10)
+    (def y 20)
+
+    ;; Expected optimization: (+ 10 20) -> 30
+    (def z (+ x y))
+
+    ;; 2. Tracking assignments (Assign-Safety)
+    ;; Inlining must be careful with 'assign'.
+    (def result
+      (do
+        (def a 100)
+        (assign a 200) 
+        ;; Expected optimization: 200
+        a))
+
+    ;; 3. Return a tuple of optimized results
+    [z result]))