add support for NCDatasets.jl - #68

The extension read and wrote through NCDatasets' CFVariable, so the CF
decoding (scale_factor, add_offset, fill values, time units) was applied
twice: once by NCDatasets and again by YAXArrays' CFDiskArray/timedecode.
A variable with scale_factor 0.5 and add_offset 100 came back as 150.5
instead of 101, and a "days since" time axis got a corrupt element type.
Wrap the raw Variable (nc[name].var) instead, as the NetCDF.jl backend
does, and copy _FillValue into missing_value so YAXArrays honours it.

Also:
- forward haschunks/eachchunk to the file (they were imported but never
  defined, so every handle fell back to a single whole-array chunk)
- iscompressed recognises Zstandard when NCDatasets provides zstandard()
- open_dataset_handle no longer masks a failed open with close(nothing)
- add_var passes deflate/zstd keywords only when requested, so zstdlevel
  never reaches a defVar that does not know it
- allow_missings is false: defVar wants a DataType, and YAXArrays handles
  Union{Missing,T} through missing_value as for NetCDF.jl
- NCDatasets compat is "0.14"; 0.12 predates the DiskArrays integration
- tests: chunking, failed-open and raw-value/attribute round trip

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>