Top-level TOML keys that legacy schema versions had but V3 either
removed or restructured. Suppresses “unknown key” warnings on V1/V2
configs flowing through migrate_to_current: every key here is
consumed by V1Config::migrate or V2Config::migrate, so it’s
expected on a stale-but-being-migrated config.
Walk a TOML tree and encrypt every string leaf whose terminal key
name appears in [secret_key_names]. Strings already in enc2: /
enc: form are left alone (idempotent). Arrays of strings under a
matching key (e.g. paired_tokens) are encrypted element-wise.
Pure migration from any supported version’s TOML string into the current
schema version’s TOML string. Returns Ok(None) when the input is already
at CURRENT_SCHEMA_VERSION.
File-API wrapper: read disk config, migrate, write <file>.backup
adjacent to the original, then atomically replace the original. Returns
Ok(None) when already current.