ledgr_param() creates a serializable placeholder used in parameterized
feature declarations. It is resolved with concrete feature parameter values
before precompute, run, or sweep execution.
Details
First-pass support is limited to scalar tuning arguments in
ledgr_ind_sma(), ledgr_ind_ema(), ledgr_ind_rsi(),
ledgr_ind_returns(), ledgr_ind_ttr(), and
ledgr_ind_ttr_outputs(). Custom ledgr_indicator() construction remains
concrete-only.