docs: v6.6.3 catch-up - #69

Docs-only PR adding accurate, well-sourced coverage of v6.6.3 freeze mode and the new frozen-rpc-router binary; I verified flag names, defaults, error codes/messages, routing rules, header behavior, the make target, and both cross-page anchors against the four source sei-chain PRs and found no inaccuracies. Remaining notes are wording/completeness suggestions plus the fact that both second-opinion review passes produced no output.