family: document Misc-vs-IPC boundary rule; keep robust_list in Misc (task 520)

Resolve the get_robust_list/set_robust_list classification decision: keep
both as FamilyMisc rather than promoting them to FamilyIPC alongside the
recently-moved futex_* syscalls.

Rule (now documented in family.go next to the futex IPC block): a syscall
is IPC only if it PERFORMS the actual IPC/sync operation (futex
wait/wake/requeue on the futex word, or an op on an IPC object). Per-thread
registration/bookkeeping that merely hands the kernel a pointer it consults
later -- rseq and get_robust_list/set_robust_list -- stays Misc. man 2
get_robust_list confirms the robust futex list is 'managed in user space:
the kernel knows only about the location of the head'; these syscalls
register/query that per-thread head pointer and never wait/wake or touch
shared memory, structurally identical to rseq. The split axis is
operation-vs-registration, not name similarity.

No classification change, so mage generate is a no-op (generated artifacts
and docs/syscall-tracing-plan.md unchanged). Strengthened the rseq/
robust_list comments in family_test.go and the TestClassifyGetRobustListPidNotFd
lock-in comment in classify_test.go to cite the rule and mark the decision
resolved.

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

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