docs(zarr-metadata): a changelog fragment for the codec pipeline

Assisted-by: ClaudeCode:claude-opus-5-5

Author: d-v-b

packages/zarr-metadata/changes/+layer-4c.feature.md

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+**Breaking:** a v3 array's `codecs` are read as a pipeline: in order --
+array -> array codecs, then one array -> bytes codec, then bytes -> bytes
+codecs -- and each codec judged against the chunk it is handed, which is
+the grid's chunks of the array's data type, as the array -> array codecs
+before it hand them on. A codec out of order, a second array -> bytes
+codec or none at all; a `bytes` codec without an `endian` handed numbers
+of several bytes, or handed values that vary in size; a `transpose` whose
+`order` has another number of axes than its chunk; a `cast_value` to or
+from a data type that models no real numbers, wrapping to one that is
+not integral, or mapping a scalar that is not a fill value of the data
+type on its side; a `scale_offset` handed values that are no numbers, or
+with an `offset` or `scale` that is not a value of its data type; and a
+struct field whose values vary in size are each a problem where they
+sit, where the package accepted them before. `read_pipeline(codecs,
+chunk)` gives each codec with the chunk it is handed,
+`chunk_grid_lengths(grid, shape)` the lengths a grid's chunks take, and
+`storage_of(data_type)` how a data type's values are stored. A
+definition's `rules` are handed the fields its configuration holds as
+the scope read them, and the kinds gain `chunk_lengths` (grids),
+`storage` (data types), and `chunk_rules` and `transition` (codecs).