[SizzleUnrlsd]

Summary

Phase 3 of docs/architecture.md: PyIR now follows the control-flow graph (§5, §6).

• Acceptance tests first (test: define PyIR over the CFG), implementation follows.
• The IR model carries explicit terminators: Return, Branch, Jump, Raise and ForNext, which keeps Python's iteration protocol explicit (get_iter before the loop header, for_next in it) instead of an LLVM-style switch. BasicBlock and FunctionIR are immutable; blocks mirror the CFG in its order and share its block ids.
• PyIRAnalysis requires CFGAnalysis and lowers one block at a time. Values are numbered once per function. Parameters that are never reassigned stay plain values; reassigned ones are stored to a local at entry so every block reads the same slot.
• --emit-ir prints one block per basic block, including unreachable code after return / raise.
• README documents the supported control flow.

Not in this PR: dominators, SSA and phi nodes (next Phase 3 item), exception edges, with, the remaining §6 instructions that need new PyHIR nodes (SetAttr, SetItem, BuildList, ...).

Part of #13: terminators, immutable blocks and lowering over the CFG. The remaining §6 instructions stay open there.