fix(federation): ignore retired clock keys during causal replay

[Fail-Safe]

Summary

• prevent false divergence detection when a post-migration event is causally newer on every stable Cortex ID but an older event retains a retired name-key clock component
• preserve genuine stable-ID concurrency and ambiguous legacy-clock cases
• leave historical event clocks and signatures unchanged
• bump the patch version to 0.21.2

Validation

• cargo fmt --check
• cargo clippy --all-targets -- -D warnings
• cargo test --all-targets --locked
• make release-check

The regression suite covers ordered stable clocks with a retired key, genuine stable-ID concurrency, legacy-only ambiguity, end-to-end replay materialization, and preservation of the stored historical clock.