tters - Sequential Target Trial Emulation Data Expansion (Rust + Polars Backend)

Fast, verified data-expansion stage for sequential target trial emulation, backed by a Rust and Polars engine via the 'extendr' crate. Reproduces, bit-for-bit, the expansion output of the 'TrialEmulation' R package. The heavy lifting lives in the 'tte-expand' Rust core crate; this package is a thin binding layer.