Active signer, x-only (32-byte) hex.
Deprecated signers keyed by x-only hex, mapped to their cutoff. The cutoff
is always a bigint (arkd advertises it non-nullable); 0n means "no cutoff
advertised" (→ DUE_NOW).
Read-only: the wallet hands out its live _deprecatedSigners here rather
than a copy, and that map drives coin selection and the pendingRecovery
balance bucket. A holder must not write through it.
The server's signer set, pre-normalized to x-only hex for cheap repeated lookups. Built once per migration/reporting pass from a fresh ArkadeInfo snapshot via signerSetFromInfo.