Add --block-retry-delay and schema_version

[jehrr]

• --block-retry-delay SECONDS: doubling wait before re-fetching a refused page; unset keeps the old linear wait. One page_flow.block_pause for all three engines.
• schema_version (1.0.0) in the sidecar; diff_runs refuses differing MAJORs, accepts MINOR differences and sidecars without one.
  Controls: breaking block_pause turns 'set, it starts at the base and doubles' red; diff_runs checked by hand (major refused exit 2, minor and missing accepted).

🤖 Generated with Claude Code