feat: add PR-wide risk score to diff aggregates

[stephenc222]

Summary

• Collapse hotspots diff's per-function LRS deltas into a single pr_risk_score + band for the whole PR (aggregates.pr_summary in JSON/JSONL, a line in text output, a summary card in the HTML report)
• pr_risk_score is the net LRS delta across the diff (new functions add their LRS, deleted functions subtract theirs, modified functions add their ΔLRS); band is the worst risk band reached by any new/modified function
• Checked hotspots-research's promotion tracker first: "Review Hotspots / PR-level triage" has no validated finding requiring a promotion brief — this only rolls up already-computed per-function LRS values, it doesn't introduce a new predictive signal

Test plan

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test (full workspace, including new test_diff_pr_risk_summary_collapses_all_changes_to_one_score)
• Manual smoke test: hotspots diff HEAD~5 HEAD --auto-analyze in both --format json and --format text

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