Convert parsed EDTF structs into a {start_date, end_date} tuple of Date.t() values suitable for SQL- or Ecto-style range queries.

The user-facing entry point is EDTF.to_date_range/1, which accepts an EDTF string and delegates here once parsing has succeeded. This module can also be called directly with an already-parsed struct.

Semantics

• Bounded inputs yield two concrete Date.t() values.
• Explicitly open inputs (/.., ../, [..2020], [2020..]) yield :unbounded on the open side.
• Inputs with an unknown bound (1985/, /1985) yield :unknown on that side.
• Qualifiers (~, ?, %) are ignored — the range uses the nominal date.
• Unspecified digits (X) are expanded to their full place-value span when they form a contiguous suffix of a component (e.g. 19XX → 1900-01-01 to 1999-12-31). Non-suffix unspecified digits return {:error, :unsupported}.
• Seasons map to month ranges (quarters, quadrimesters, semesters are unambiguous; codes 21–24 are treated as northern-hemisphere; Winter and southern-hemisphere Summer span the year boundary).