feat(runs): add pure run snapshot projector

[solomonneas]

Summary

• Add a pure, deterministic projector that derives five journal-backed fields while preserving all 44 current run.json fields.
• Fail closed on unknown snapshot fields, invalid event chains, unmapped statuses, and unencodable values, with golden replay and regression coverage.

Scope

This is slice 3 of #568. It defines and tests the projection contract without wiring the projector into writers, recovery, or CLI paths.

Verification

• ./scripts/verify
• 4,864 passed, 3 skipped, 82.95% coverage
• 132 focused projector, event, journal, and lifecycle tests passed

Part of #568.

[coderabbitai]

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