Maps the base parameter (rec_pars, log_M1,
log_growth_pars) out of estimation only for stratum groups
whose linkage formula holds no intercept. With an intercept
in the formula (~ 1, ~ temp, ...) and a nonzero est_phase the
base parameter holds the level and stays estimable; the linkage
(Intercept) row is fixed at 0 instead. When an intercept row has
est_phase == 0 (the documented "fix at init" contract), the base
parameter is mapped off as well, so a fixed intercept truly fixes the
parameter at its build_params() initial value. Slope-only formulas
(~ 0 + temp) emit no intercept row, so we mask the base parameter to
keep it at its build_params() default and let the slope rows define
the year-by-year offset.