- Drop `type='ipv6'` rows whose bracket-stripped inner part contains
no `:`, since every valid IPv6 textual form has at least one
- Cover the offender (`[]:8776`), bracketed garbage (`[abc]:8776`),
the unspecified (`::`), loopback, and full forms in tests
- Keep `dns` and `ipv4` rows untouched
Migration 8 retyped any `[..]:N` dns row to ipv6 by bracket pattern
alone, without checking the inner part is parseable
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