[v0.3] Add optional CSV export for findings and warnings

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Summary

• add an optional CSV output mode for findings and warnings
• keep existing Markdown and JSON outputs unchanged
• make it easier to post-process LogLens results in spreadsheets or lightweight analysis pipelines

Scope

• add CSV export as an optional output alongside existing report.md and report.json
• include findings and warnings in a stable, documented schema
• keep detector thresholds and parser behavior unchanged
• do not add enrichment, cross-host correlation, or SIEM-like behavior
• do not remove or replace existing output formats

Acceptance Criteria

• LogLens can emit CSV output when explicitly requested
• CSV schema is documented and stable enough for simple downstream use
• existing report.md / report.json outputs remain unchanged by default
• regression tests cover the CSV contract for at least one single-host and one multi-host sanitized fixture
• no detector threshold or parser family behavior changes are introduced in this issue