Build explicit control-flow graphs #11

Each function becomes a CFG of basic blocks ending in an explicit terminator.

• Terminators: Branch, Jump, Return, Raise, plus ForEach to keep Python's iteration protocol explicit
• Predecessors, successors, reachability, graph integrity checks
• Exception edges for try/except/finally (feat: try with exception edges, await and yield #36; finally on the normal path only)
• Loops and back edges
• if/else, elif, early returns, while, for, break, continue, raise. with and try follow their PyHIR nodes

Reference: docs/architecture.md §5 CFG.

Delivery: #12, #36 (exception edges)