test(gettid): lock in KindNull/FamilyProcess/UNCLASSIFIED classification

Audit of gettid(2) ('pid_t gettid(void)', no args, always succeeds) found
the classification correct and consistent with its no-arg id-returning
siblings getpid/getppid/getuid/getgid (FamilyProcess, KindNull enter,
ret_event UNCLASSIFIED exit), and mage generate produces no diff. However
gettid lacked dedicated lock-in coverage and was missing entirely from the
family_test.go Process table despite its siblings being asserted there.

Add TestClassifyGettidNullEnter and TestClassifyExitGettidUnclassifiedRet
(mirroring the getgid pattern: enter null_event capturing nothing, exit ret
classified UNCLASSIFIED so the returned tid is never mistaken for a byte
count) plus gettid enter+exit FamilyProcess assertions in family_test.go.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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