enrollcast 0.1.0

• Initial CRAN release.
• progression_ratios() computes progression ratios from historical
  grade-level enrollment, with mean, geometric, median, last, and weighted
  summaries. It warns about calendar-year gaps across the complete supplied
  history before n_years selects the most recent transitions.
• progression_matrix() places ratios on the sub-diagonal of a square
  projection matrix, leaving the entry-grade row at zero.
• project_enrollment() advances a base enrollment vector over an arbitrary
  horizon, from either ratios or a prebuilt schedule, overwriting the exogenous
  entry grade each year. Missing schedule coefficients are preserved with an
  aggregate warning rather than imputed.
• swing_schedule() builds per-year schedules that hold enrollment flat during
  swing years, apply recovery multipliers, then resume normal projection.