Feasibility helpers for sparse MET allocation

Utility functions for assessing whether a proposed sparse MET configuration has sufficient per-environment capacity to assign every non-common treatment at least once, and for deriving safe values for n_test_entries_per_environment prior to calling allocate_sparse_met().

Details

These helpers address a specific failure mode: passing n_test_entries_per_environment that is too small to cover all treatments, which causes allocate_sparse_met() to either error, produce incomplete allocation, or require allow_approximate = TRUE in ways that leave some treatments unassigned. Calling these helpers before allocation lets the user verify feasibility and choose a safe capacity without trial and error.

The three exported helpers serve distinct purposes. min_k_for_full_coverage() computes the arithmetic minimum. suggest_safe_k() adds a user-defined buffer on top of that minimum and returns a single ready-to-use integer. warn_if_k_too_small() checks an existing configuration and emits a warning with a corrective suggestion if the capacity is insufficient. The internal function .check_full_coverage_feasibility() is called by allocate_sparse_met() to enforce the same condition with a hard stop.