Membership was a linear scan over a slice, and ensureCorrectWritables runs it
for every volume on every heartbeat, so the master's steady-state cost per
volume server is quadratic in that server's volume count.
BenchmarkSyncDataNodeRegistration, median of 3:
1000 volumes 565.7us -> 535.0us -5.4%
100000 volumes 1.665s -> 55.3ms -96.7%
Allocations are unchanged at both sizes, so the difference is the scan.
Claude-Session: https://claude.ai/code/session_01P3pE6J2UPFp6G3ksfMV4s1