Add block scoping and statement/expression distinction

Implement strict block scoping and distinguish between statements and
expressions.
This change introduces hierarchical scopes for functions and blocks,
enforces that `def` is a statement with no return value, and prevents
statements from appearing in expression positions or as the last element
of a block. A new example `design-flaw.myc` is added to demonstrate
the uninitialized variable issue solved by these changes.

docs/binder-refactoring-log.md

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+# Development Log: Binder Refactoring & Block Scoping
+
+## Status Quo
+- Myc currently uses a flat scope per function.
+- `def` is treated as a side-effect expression that "leaks" into the function scope regardless of nesting (e.g., inside `if` or `do`).
+- This leads to uninitialized variables at runtime if the definition is skipped by control flow.
+
+## Goal
+Implement strict **Block Scoping** and distinguish between **Statements** and **Expressions** to eliminate undefined variable states.
+
+### Rules
+1. **Block Scoping:** Every `(do ...)` and `if` branch creates a new lexical scope.
+2. **Statements vs. Expressions:** 
+    - `def` is a **Statement**.
+    - Statements have no return value (Void).
+    - Statements **cannot** be used as expressions (e.g., as arguments, RHS of assignments, or conditions).
+    - Statements **cannot** be the last element of a block (since the block's value is determined by its last expression).
+3. **No Shadowing:** Redefining a symbol within the **same** scope level is an error. Shadowing from outer scopes is allowed (standard lexical scoping).
+4. **Self-Reference:** A variable is only available in its scope **after** its definition is complete (RHS of `def` sees outer scope).
+
+## 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 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 3: VM Compatibility
+1. [ ] Ensure the VM handles "Void" results from statements correctly.
+2. [ ] (Optional) Optimize stack allocation based on scope depth.
+
+---
+
+## Log Entries
+
+### 2024-05-22: Initialization
+- Analyzed `design-flaw.myc`.
+- Defined the "Statement" concept to solve the uninitialized variable problem.
+- Initialized this log.