Python shared CFG: optimize explicit-step binding order

[yoff]

Summary

Introduce a private explicitAfterValue wrapper with bindingset[ast, successor] and pragma[inline_late], and use it only within explicitStep. This binds caller AST/successor demand before control-flow-node expansion. CFG semantics are unchanged; NodeImpl.isAfterValue is untouched.

This draft is stacked directly on yoff-python-cache-cfg-node-map at exact base SHA 133845e41556803743fbc1f76e7fce54941f20a1.

Validation

This is DCA-only aggregate validation, not a merge proposal.

• Airflow: -48,951,260 combined joins (-4.379%).
• Nova: -38,510,549 combined joins (-3.563%).
• Salt: -74,634,190 combined joins (-5.486%).
• Exact internal relations were equal for Rank6ExplicitStep, Rank6Succ, Rank6BlockMembership, Rank6BlockSucc, Rank6Dominates, Rank6Reachable, Rank6SsaReads, Rank6SsaWrites, and Rank6SsaPhi; target BQRS output was equal.
• No new materialization was introduced.
• Airflow clean-cache reproduction produced equal logical bytes.

Local validation:

• codeql query format --check-only shared/controlflow/codeql/controlflow/ControlFlowGraph.qll
• ExceptionReachabilityTest.ql
• SsaTest.ql
• Python shared-CFG dominance and evaluation-order suites (29 tests total; all passed)

This draft preserves the DCA experiment as a clean rank-6 stack layer and is not proposed for merge.