docs: add PRP-27 scenario simulation full version (#223)

[w7-mgfcode]

Adds the PRP for the Full Version of Scenario Simulation / What-If Planning — a strict increment on the shipped MVP scenarios slice (PRP-26).

Closes #223.

Doc-only change: PRPs/PRP-27-scenario-simulation-full-version.md + supporting PRPs/ai_docs/exogenous-regressor-forecasting.md. No code touched.

Covers 4 dependency-ordered phases — leakage-safe future feature-frame generator, exogenous-regressor forecasting (HistGradientBoostingRegressor, no new dependency), scenario library + multi-scenario comparison, and an approval-gated agent save_scenario tool. PRP passed the prp-quality-agent gate (READY).

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