refactor(fe): expose regression feature builders as standalone, streaming-friendly components shared by local + global lanes

[sipemu]

Context

The pipeline has two regression feature-engineering implementations that overlap heavily but diverge in capability:

• Local regression method → anofox-forecast's RegressionForecaster FE: lags + auto_lags (AIC/BIC), trend_type (linear/quadratic/cubic/exponential/logistic/theil_sen), differencing/seasonal_differencing, fourier, the rolling family, exog_features (lags + rolling on exog), event_distance (steps-since/until a holiday/promo), WLS.
• Global lane (anofox-orchestration::global_regression, a separate reimplementation, fix: rename MetricType to TsMetricType (#185) #186): lags (no auto), the rolling family, ewm, calendar Fourier/one-hot, contemporaneous exog, static columns, LOO panel mean/std, per-series scaling (dynamic/ytd), yeo-johnson, trend_decompose, per-model transforms.

They re-code the common FE (lags/rolling/fourier) independently, and the global lane lacks several rich local features — notably auto_lags, trend_type, exog-FE, and event_distance — that directly target the seasonal/cumulative-YTD gaps in the #366 roadmap.

Why the split exists

• The local FE is coupled to RegressionForecaster's per-series fit (it owns a materialized design matrix), so it isn't callable from the global lane's panel-pooled, streaming-Gram fit (which accumulates XᵀX without materializing the 569k×p matrix, and needs cross-series-comparable features).

Ask (structural)

Expose anofox-forecast's regression feature engineering as standalone, streaming-friendly feature builders — pure (series, t, cfg) -> feature row functions decoupled from the RegressionForecaster fit — so both the local per-series fit and the global panel/streaming fit can call the same builders. This:

• eliminates the duplicate lag/rolling/fourier code,
• instantly gives the global lane auto_lags / trend_type / exog-FE / event_distance,
• keeps the streaming-Gram constraint (the builder writes into a reusable scratch row; the panel fit visits rows one at a time).

Larger than the incremental ports (#386/#388/event_distance), but the principled fix.

Filed from anofox-orchestration; the builder extraction lives in anofox-forecast.