Ignore observational reads in replay receipt validation

[mikamikasuki]

Summary

• Exclude read-only public-context reads from the replay validator's scientific action receipt check.
• Keep validation of unregistered or forged scientific transitions unchanged.
• Add a regression test confirming an otherwise valid replay remains valid when its history includes an observational context read.

Validation

• pytest tests/test_mechanism_replay.py: 62 passed.