docs: v6.6.3 catch-up - #69

Well-structured docs-only addition covering freeze mode and the new frozen-rpc-router; I verified the internal links (/node/snapshot, /node/node-operators#default-configurations), the anchor #frozen-rpc-router, heading-level nesting, sentence-case headings per AGENTS.md/Vale, and that freeze-height already exists in the generated app.toml at node/node-operators.mdx:172, so the cross-reference resolves. No blockers, but a few accuracy claims (the make build target, eth_getLogs toBlock-only routing) should be confirmed against the source PRs before merge.