Add shared runtime session affinity

[NavinAgrawal]

Summary

• add pure session-affinity and state-placement contract for P3 shared runtime planning
• document local-only, shared-eligible stateless, and forbidden upstream classes
• extend shared-runtime guardrails journey contract

Verification

• private: make precommit PYTEST_WORKERS=0 (1396 unit, 152 journey, 0 failed, 0 skipped)
• private: make maintainer-violations PYTEST_WORKERS=0 (0 violations, 12 advisories)
• private: make maintainer-grade-quality PYTEST_WORKERS=0 (A+ 100.0)
• public: make precommit PYTEST_WORKERS=0 (1396 unit, 118 journey, 0 failed, 0 skipped)
• public: make maintainer-violations PYTEST_WORKERS=0 (0 violations, 12 advisories)
• public: make maintainer-grade-quality PYTEST_WORKERS=0 (A+ 100.0)

Mutation was not run locally to respect Mac load governance.