Refactor Binder to use new scope structure

This commit refactors the Binder implementation to better align with the
updated scope structure.
Key changes include:

- Introducing `ExprContext` to distinguish between statement and
  expression contexts.
- Modifying `define_variable` to handle both global and local scope
  definitions more robustly.
- Updating `resolve_variable` to correctly handle upvalue captures,
  especially for global addresses.
- Adjusting `bind` and related functions to use the new `ExprContext`.
- Updating the refactoring log to reflect completed phases.

docs/binder-refactoring-log.md

@@ -20,16 +20,16 @@ Implement strict **Block Scoping** and distinguish between **Statements** and **
 
 ## Implementation Plan
 
-### Phase 1: Binder Infrastructure (In Progress)
-1. [ ] Update `CompilerScope` to support hierarchical nesting via a stack in `FunctionCompiler`.
-2. [ ] Implement `push_scope` and `pop_scope` in `Binder`.
-3. [ ] Refactor `resolve_variable` to traverse the scope stack (inner-to-outer).
-4. [ ] Update `define_variable` to enforce "No Shadowing" at the current scope level.
+### Phase 1: Binder Infrastructure (Completed)
+1. [x] Update `CompilerScope` to support hierarchical nesting via a stack in `FunctionCompiler`.
+2. [x] Implement `push_scope` and `pop_scope` in `Binder`.
+3. [x] Refactor `resolve_variable` to traverse the scope stack (inner-to-outer).
+4. [x] Update `define_variable` to enforce "No Shadowing" at the current scope level.
 
-### Phase 2: Statement/Expression Validation
-1. [ ] Introduce a mechanism to track "Context" (Expression vs. Statement) during binding.
-2. [ ] Validate that `def` is only used in statement positions.
-3. [ ] Enforce that the last node in a `BoundKind::Block` is an expression.
+### Phase 2: Statement/Expression Validation (Completed)
+1. [x] Introduce a mechanism to track "Context" (Expression vs. Statement) during binding.
+2. [x] Validate that `def` is only used in statement positions.
+3. [x] Enforce that the last node in a `BoundKind::Block` is an expression.
 
 ### Phase 3: VM Compatibility
 1. [ ] Ensure the VM handles "Void" results from statements correctly.
@@ -43,3 +43,10 @@ Implement strict **Block Scoping** and distinguish between **Statements** and **
 - Analyzed `design-flaw.myc`.
 - Defined the "Statement" concept to solve the uninitialized variable problem.
 - Initialized this log.
+
+### Phase 1 & 2 Completed
+- Introduced scope stack into `FunctionCompiler`.
+- Introduced `ExprContext` (`Expression` / `Statement`) in the `Binder`.
+- `def` now acts strictly as a statement.
+- Variables defined inside nested scopes (e.g. inside `if` or `do`) do not leak into outer local scopes anymore.
+- Outstanding behavior to discuss: `def` inside nested scopes at the *root* level still leaks into globals.