Assert adb find loop-affinity invariant at lifetime entry points

Assert adb find loop-affinity invariant at lifetime entry points

The dns_adbfind_t lifetime model has no reference counting; storage
liveness is held together by find->lock and the FIND_EVENT_SENT
idempotency flag, plus an unwritten cross-module rule that all
non-trivial operations on a find run on find->loop. If a caller
violates that rule, the unlock-relock window in dns_adb_cancelfind
(and similar paths) becomes a use-after-free and we crash later
inside libpthread on a corrupted mutex.

Add REQUIREs at dns_adb_cancelfind, dns_adb_destroyfind and
find_sendevent so a violation aborts at the offending call site
rather than silently freeing storage another loop is still touching.
Also poison find->magic with ~DNS_ADBFIND_MAGIC in free_adbfind so
DNS_ADBFIND_VALID catches reuse-after-free at the next public entry
point instead of letting the dangling pointer reach the mutex code.

Assisted-by: Claude:claude-opus-4-7