Function next_issue_due 

pub fn next_issue_due(
    center: AnalysisCenter,
    product_type: ProductType,
    now: ProductDateTime,
) -> Result<NominalIssue, DataCatalogError>

Return the next catalog issue nominally due at or after now.

This is a pure catalog query. It performs no listing fetch and does not say whether an archive has published the issue. The schedule rules are pinned to the IGS product descriptions, the IGS Analysis Center Coordinator schedule, and the relevant center descriptions in the committed nominal schedule provenance fixture.

Rules represented here:

• IGS combined ultra-rapid SP3 is released 27 hours after the filename’s coverage start: 24 observed hours followed by the 03:00, 09:00, 15:00, or 21:00 UTC release. Analysis-center ultra products use the 26 h 50 min submission deadline. A two-day ultra identity names a 24-hour observed interval followed by a 24-hour predicted interval.
• GFZ rapid SP3 and CLK use the daily 15:45 UTC analysis-center deadline for the preceding UTC day. CODE rapid IONEX uses the published less-than-24 h latency boundary, represented as 00:00 UTC following the map date.
• Final SP3 and CLK lines represent the newest daily identity in each weekly batch. Analysis-center batches are due Wednesday at 05:00 UTC, 11 days after the GPS week ends. The IGS combined final batch is due by Friday, represented as 23:59:59 UTC, 13 days after the week ends. Final IONEX uses the published approximately 11-day weekly latency, with a date-only deadline represented as 23:59:59 UTC.
• CODE predicted IONEX is due at 00:00 UTC one or two days before its map date, matching the line’s cataloged prediction horizon.

Near-real-time WUM and broadcast navigation lines are outside these pinned rules and return DataCatalogError::UnsupportedNominalSchedule.