feat: add ASTTransformer for in-place AST node replacement

[boolangery]

Problem

The existing ASTVisitor and ASTRecursiveVisitor only support read-only traversal. There is no way to transform or replace AST nodes during traversal, which is needed for tasks like:

• Constant folding / expression simplification
• Variable renaming
• Code deobfuscation
• AST-based optimization passes

Solution

Adds ASTTransformer, a production-grade AST visitor that supports in-place node replacement.

Design

• Iterative stack-based traversal — avoids recursion limits on deep trees
• Direct parent tracking via (key, container) tuples — O(1) replacement, no brute-force key search
• Replacement children visited — after replacing a node, children of the new node are traversed
• Identity checking — replacement is not node avoids unnecessary mutation
• Root replacement — the transformed root is returned from visit()

Usage

class NumberDoubler(ASTTransformer):
    def visit_Number(self, node):
        return Number(node.n * 2)

tree = ast.parse("x = 5")
new_tree = NumberDoubler().visit(tree)

Comparison with PR #74

This implementation improves on the original PR by:

• O(1) parent replacement via tracked keys (vs O(n×m) brute-force dict search)
• Proper root replacement support
• Visiting replacement node's children (not stale original children)
• Comprehensive test suite (15 tests)
• No commented-out debug code
• Clean docstrings

Tests

15 tests covering:

• Value replacement, statement replacement, deep nesting
• Root replacement, recursive constant folding
• Round-trip identity, None/no-op semantics
• List child iteration, parent class dispatch

Closes #74