Drop stale syncer pushes before merge

[WaldenLee2005]

Summary

• drop stale PUSH_FRAGMENT responses whose base_version does not match the syncer's current fragment version for all wire dtypes
• keep q4 reconstruction behind the same freshness gate and handle all-stale rounds with a no-op rebroadcast instead of committing stale values
• add unit coverage plus a protocol-level manual stale-push check
• no docs changes included

Tests

• cargo test stale
• python -m py_compile scripts/manual_stale_push_check.py
• python scripts/manual_stale_push_check.py