quantmodel — Add single-asset substitution backtest reruns

Objective

Add single-asset substitution backtest reruns.

Notes: Added fixed-signal single-holding substitution reruns with identical common-window baseline, explicit substitution/as-of/execution/cost assumptions, deterministic validation, focused tests, and durable contract notes.

Approach

Reuses existing report research/findings/asset-substitution-t253.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 t253 in sampled log [reconstructed — uncertain].
Source report: research/findings/asset-substitution-t253.md
Task-history entry: research/findings/task-history.json#t253

Results

Conclusions and metrics are in research/findings/asset-substitution-t253.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.