feat(parser): add util-linux login handler

[stacknil]

Summary

• add a dedicated handler for selected upstream util-linux login failure and session messages
• keep terminal/host FROM fields out of source_ip, and classify unsupported shapes as login_other
• expand the sanitized mixed auth corpus from 150 to 160 lines with locked coverage telemetry
• add syslog/journal, malformed-shape, 7-handler permutation, and fuzz-seed coverage

Validation

• Windows MSVC build and CTest: 6/6 passed
• WSL/GCC clean build and CTest: 6/6 passed
• actionlint on all workflows
• staged diff, JSON/corpus contract, local Markdown link, privacy/secret, and fixture-IP scans

Evidence boundary

The selected message shapes are anchored to the pinned upstream util-linux login.c syslog strings. LogLens does not infer a network source IP from the FROM terminal/host field, and unsupported/localized variants remain visible parser warnings.