Tests were dumping megabytes of zerolog output on failure; silence
at init and let individual tests opt in via SetGlobalLevel when they need
log-driven assertions.
Updates #3157
Add hscontrol/util/zlog package with:
- zf subpackage: field name constants for compile-time safety
- SafeHostinfo: wrapper that redacts device fingerprinting data
- SafeMapRequest: wrapper that redacts client endpoints
The zf (zerolog fields) subpackage provides short constant names
(e.g., zf.NodeID instead of inline "node.id" strings) ensuring
consistent field naming across all log statements.
Security considerations:
- SafeHostinfo never logs: OSVersion, DeviceModel, DistroName
- SafeMapRequest only logs endpoint counts, not actual IPs
