[v0.3] Add host-level summaries to report.md and report.json

[stacknil]

Summary

• add concise per-host summary sections to reports when input contains multiple hosts
• keep existing detector thresholds and finding logic unchanged
• make multi-host inputs easier to review without drifting into SIEM-like correlation

Scope

• summarize parsed events, findings, and warnings by hostname
• preserve existing top-level summary and golden contract discipline
• update golden outputs / regression fixtures accordingly
• no enrichment, no cross-host correlation logic, no detector-policy changes

Acceptance Criteria

• report.md includes a compact host summary section when more than one hostname is present
• report.json includes a stable host-summary object or array
• existing global summary and finding objects remain backward-compatible where practical
• golden report contract tests are updated and remain deterministic