[v0.2] Expand parser fixture matrix with more sanitized sshd and pam_unix variants

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Summary

Expand the parser test/fixture matrix with more realistic but sanitized sshd and pam_unix variants so parser behavior is exercised against a broader set of common authentication log patterns.

Scope

• add more representative sshd variants that stay public-safe and fully sanitized
• add more pam_unix auth/session variants that are common in Linux auth logs
• preserve placeholder hosts, IPs, usernames, and commands
• keep parser coverage deterministic and easy to understand

Acceptance Criteria

• fixture coverage includes additional real-world-style sshd and pam_unix variants
• new fixtures remain public-safe and do not include real infrastructure identifiers
• parser tests cover the new variants and clearly account for recognized vs unrecognized lines
• README or docs are updated if supported patterns materially change