fix(network): complete deterministic replay acceptance

[darwin67]

Summary

• finalize RFD 2 acceptance evidence and mark the proposal committed
• cover replay compatibility for every network identity class
• remove preparation-only fixture inputs before atomic publication
• document measured local and GitHub-hosted acceptance results, limitations, and the next-RFD recommendation

Oracle review

Oracle found one P2 issue: published preparation fixtures were outside artifact digest validation. The fixture directory is now removed before publication and covered by a regression. A focused follow-up review reported no remaining or newly introduced findings.

Verification

• nix --extra-experimental-features 'nix-command flakes' flake check
• .agents/dev cargo fmt --all -- --check
• .agents/dev cargo clippy --locked --workspace --all-targets --all-features -- --deny warnings
• .agents/dev cargo test --locked --workspace --all-targets --all-features
• make check-rfds
• bash -n .agents/dev .agents/setup scripts/*.sh
• .agents/dev ./scripts/phase4-acceptance-test.sh
• .agents/dev ./scripts/phase4-acceptance.sh