Document Process-vs-Misc boundary; keep set_tid_address in Process (task 920)

Resolve the family-split question for set_tid_address vs its per-thread
registration siblings rseq/set_robust_list/get_robust_list (Misc).

DECISION: keep set_tid_address in FamilyProcess. The 520 registration-vs-
operation rule governs the IPC-vs-Misc boundary (does the syscall perform the
futex/sync op?), not Process-vs-Misc. set_tid_address registers
clear_child_tid, the kernel's primary thread-EXIT notification mechanism
(zeroed + FUTEX_WAKEd at teardown), set by the C runtime for essentially every
thread via clone(2) CLONE_CHILD_CLEARTID, and returns the caller's tid like
gettid/getpid -- mandatory thread-lifecycle plumbing belonging with
clone/fork/exit/gettid. rseq (scheduling optimization) and robust_list (opt-in
futex cleanup) are OPTIONAL per-thread features and stay Misc.

- family.go: add Process-vs-Misc boundary-rule block (mandatory-thread-
  lifecycle vs optional-opt-in-feature axis) next to set_tid_address.
- family_test.go: lock in set_tid_address enter+exit as Process with a WHY
  comment contrasting it against the rseq/robust_list Misc cluster.

No generated-artifact or docs drift: classification unchanged (Process list in
docs/syscall-tracing-plan.md already correct); mage generate is idempotent.

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

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