ettbc (development version)

Added clone_censor(): implements the clone-censor step of the target trial emulation methodology. Creates two clones per participant (STOPBASE and CONTINUE arms) and applies the corresponding censoring rules (#1).
Added expand_to_long(): converts cloned data to one row per participant-arm-month for use in discrete-time survival analysis (#1).
Added example synthetic datasets: cohort, screening_mammograms, and diagnostic_mammograms (#1).
Added vignette article “Using ettbc: Emulating a Target Trial for Breast Cancer Screening” (#1).
Updated DESCRIPTION title and description to reflect the package purpose.

ettbc 0.0.0.9000