feat(zarr-metadata)!: replace scope update with copy(update=...) and add definitions with union - #4598

…add definitions with union

`ZarrV3Context.update(**slots)` and `ZarrV2Context.update(**slots)` read
as a merge, but replace each slot whole, so passing one new definition
silently dropped every default. They become `copy(*, update=None)`,
after pydantic's `model_copy(update=...)`, whose replace-whole meaning
pydantic users already know. Adding definitions is `union`, which keeps
every definition the scope has and refuses a conflicting one; the docs,
docstrings and call sites now use it instead of splicing a slot's
definitions back in.

Fixes #424

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

…r-9bd090

Main renamed the scopes to *_CONTEXT and fields to declarations, and
added scope update call sites (ZARR_PYTHON_CONTEXT, fixed_length_utf32,
the vlen codecs). Conflicts take main's side, then rewrite each scope
update into union (when it splices a slot's definitions back in) or
copy(update=...), and restore this branch's copy/union docs under the
new names.

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