license: record segmentio/asm as MIT-0, not MIT

segmentio/asm is licensed MIT-0 ("MIT No Attribution") — standard MIT minus the attribution clause, hence strictly more permissive. go-licenses reports Unknown because the clause-trimmed text doesn't clear the classifier's MIT template threshold (and MIT-0 isn't matched either).

The earlier override mapped it to MIT, which passed but mislabeled it. Record the accurate SPDX: add MIT-0 to .allowed-licenses (it imposes no obligations, clearly acceptable) and point the override at MIT-0.