refactor(quantreg): type the demeaned-data lookup as DemeanedData

The lookup has held array-native DemeanedData entries since the shared
demean cache stopped converting to DataFrames; the annotation still
said pd.DataFrame. Annotation only, no runtime change.

Author: s3alfisc

pyfixest/estimation/quantreg/QuantregMulti.py

@@ -11,6 +11,7 @@
 
 from pyfixest.demeaners import AnyDemeaner
 from pyfixest.estimation.formula.parse import Formula as FixestFormula
+from pyfixest.estimation.internals.demean_ import DemeanedData
 from pyfixest.estimation.internals.literals import (
     QuantregMethodOptions,
     QuantregMultiOptions,
@@ -35,7 +36,7 @@ def __init__(
         weights: str | None,
         weights_type: str | None,
         collin_tol: float,
-        lookup_demeaned_data: dict[frozenset[int], pd.DataFrame],
+        lookup_demeaned_data: dict[frozenset[int], DemeanedData],
         solver: SolverOptions = "np.linalg.solve",
         demeaner: AnyDemeaner | None = None,
         store_data: bool = True,