feerecipient: document hardened sign and fetch behavior

[pinebit]

Summary

• Document that feerecipient fetch merges newly fetched registrations into the existing overrides file (keeping the latest timestamp per validator) instead of overwriting it, and that unrelated validators' entries are preserved. A fetched registration that is not newer than the existing override is discarded with a warning.
• Document that a fetch with insufficient quorum logs and returns without touching the overrides file, rather than erroring.
• Document that fetched registrations are signature-verified, and that a registration failing verification is skipped and logged while the rest of the batch is still merged and written.
• Document that a corrupt or invalid overrides file does not block fetching (it is rebuilt), and that the file is written atomically.
• Document feerecipient sign validation: the fee recipient must not be the zero address, a mixed-case address must match its EIP-55 checksum, and a manually supplied --timestamp must be later than the registration that currently has quorum on the remote API (the command rejects it otherwise). When joining an in-progress registration, its timestamp and gas limit are adopted with a warning if explicit flags are overridden.
• Document that the default gas limit is resolved from the most recent of the cluster lock, the overrides file, and the current quorum registration on the remote API.
• Add guidance for troubleshooting a validator that still shows its previous fee recipient after fetching.

Charon PR: ObolNetwork/charon#4571