Complete the PyIR instruction set and make IR immutable #13

PyIR must stay faithful to Python and must not reproduce LLVM's memory model (obj.foo stays get_attr).

Already covered: Const, Global, Symbol, BinaryOp, UnaryOp, Compare, Call, GetAttr, GetItem, GetIter, LoadLocal, StoreLocal, Return, Branch, Jump, Raise, ForNext.

• SetAttr, SetItem (feat: everyday syntax coverage and tolerant check #26); Copy is unnecessary with numbered values; CallMethod waits on the call graph
• BuildList, BuildTuple, BuildDict (feat: everyday syntax coverage and tolerant check #26), plus BoolOp, Assert, call keywords
• Branch, Jump, Raise, Return and ForNext terminators (feat: PyIR over the control-flow graph #14); Phi comes with SSA
• Import, Await, Yield (feat: PyIR imports and schema version #49: Import effect instruction for function-level imports; Await and Yield in feat: try with exception edges, await and yield #36; WithEnter and WithExit in feat: everyday syntax coverage and tolerant check #26, Raise in feat: PyIR over the control-flow graph #14)
• Immutable BasicBlock / FunctionIR with tuple instructions and one terminator per block (feat: PyIR over the control-flow graph #14)
• PyIR schema version (feat: PyIR imports and schema version #49: PYIR_SCHEMA_VERSION exported from coretrace_python.ir, part of the cache key)

Reference: docs/architecture.md §6 PyIR, §39 Rule 3.

Delivery: #14 (terminators, immutable blocks, lowering over the CFG)

Delivery: #26 (stores, builders, with, assert, keywords)

Delivery: #49 (Import instruction, schema version)