quantmodel — Add common-window comparison drawdown chart

Objective

Add common-window comparison drawdown chart.

Notes: Added a pure underwater-series helper, synchronized percentage drawdown chart, lifecycle cleanup, and focused tests; documented in research/findings/comparison-drawdowns-t234.md.

Approach

Reuses existing report research/findings/comparison-drawdowns-t234.md — tables/metrics preserved verbatim via source_report pointer. Body below summarizes grounded evidence from that report and adjacent artifacts.

Files / code / data changed

No commit SHA confidently associated with t234 in sampled log [reconstructed — uncertain].
Source report: research/findings/comparison-drawdowns-t234.md
Task-history entry: research/findings/task-history.json#t234

Results

Conclusions and metrics are in research/findings/comparison-drawdowns-t234.md; see detail page for preserved tables/metrics.

Conclusions

[reconstructed — uncertain] Outcome inferred from status done and task note.

Problems / follow-ups

See source report or PR discussion for follow-ups. No unsupported follow-ups fabricated.