Activates an identity from raw seed bytes, replacing (and
disposing) any previously active identity. Resolves false if
derivation failed or the activation was superseded by a newer
activation / clearIdentity() while in flight.
Does not consume the seed — the caller owns the buffer and must zero it when done (BL-48).
Disposes the active identity (if any) and clears it.
Active identity, or null. The previously active identity is disposed (key material zeroed) the moment it is replaced by a new activation or by clearIdentity — stale references fail loudly with
DisposedErrorper the core contract (BL-47).