docs(runs): plan shadow projection comparison

Define the slice 4 comparison contract, privacy boundary, readiness gate, crash-gap behavior, and digest-based idempotency. Add a RED-first implementation sequence that preserves the legacy run snapshot as the authority.

Co-Authored-By: Cursor <cursoragent@cursor.com>

Co-Authored-By: Codex <codex@openai.com>

Co-Authored-By: Claude <noreply@anthropic.com>

Author: solomonneas