Vega-Lite writer: DateTime / Time binned facet strips never match, so the labels never apply

build_binned_facet_label_expr builds its strip labels as a Vega expression comparing datum.value against a midpoint string produced by calculate_midpoint_string (src/writer/vegalite/mod.rs:879). For the temporal arms — Date, DateTime, Time — that midpoint is emitted as a quoted ISO string ('1973-06-25T00:00:00'), which never equals the serialized facet column value at runtime. No condition ever fires, so the strips fall through to datum.value and show raw values instead of the bin range.

The hephaestus writer computes the same labels from typed values (scales::bin_at_centre, joining the column's bin centre back to its bin) and is correct — that is the behaviour to match.

Start in src/writer/vegalite/mod.rs at build_binned_facet_label_expr and calculate_midpoint_string, then compare the temporal handling with the hephaestus writer's scales::bin_at_centre logic. Update the Date, DateTime, and Time label comparisons so they match serialized facet values and display bin ranges instead of raw values.