CoverageJSON encoders ignore anoffset to compute validity time #126

What happened?

For mars requests / grib messages with an anoffset set, the CovJSON t axis is computed as forecast date + step, ignoring anoffset. However, anoffset shifts the reference time backward from the forecast cycle (date/time), and step is counted from that reference time, not from the forecast cycle itself. The encoder writes anoffset into mars:metadata but never applies it when computing t.

I compared against the GRIB validityDate/validityTime for an affected field and every CovJSON t value was later than the corresponding GRIB validity time by exactly anoffset hours, consistently across all steps.

mars:metadata correctly reports Forecast date and anoffset, so I imagine the fix should just be applying t = (Forecast date - anoffset) + step instead of t = Forecast date + step whenever anoffset is present.

What are the steps to reproduce the bug?

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Version

develop

Platform (OS and architecture)

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Relevant log output

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