fix: preserve all hostnames per IP and write hosts file atomically

[laurentiu021]

Summary

Two P1 reliability fixes in HostsFileService.

Fixes

• Data loss (hosts-05): ReadHostsAsync took only tokens[1] (the first hostname) from each line, so 127.0.0.1 a b c lost b/c on a read→save round trip. Now emits one HostsEntry per hostname so all survive.
• Non-atomic write (hosts-09): SaveHosts called File.WriteAllLines directly over the live hosts file; a crash mid-write could leave it empty/truncated. Now writes to hosts.sysmanager.tmp then File.Move(..., overwrite: true) for an atomic replace, with temp cleanup in finally.

Tests (the service previously had only backup/restore coverage)

• ReadHostsAsync_MultipleHostnamesPerIp_AllPreserved — asserts alpha/beta/gamma all round-trip.
• SaveHosts_LeavesNoTempFileBehind — asserts the atomic-write temp file is gone after save and content is correct.
  Both use the existing path-injection NewServiceWithTempHosts helper (no real System32 hosts touched, no admin).

Verification

• Build: main + Tests 0 warnings / 0 errors.

fix: → patch release (1.20.24). Protocol C to follow.