fix: stop emitting UnstableSpecificationWarning for the spec'd "struct" data type

[d-v-b]

Summary

The struct data type now has a stable Zarr V3 specification in zarr-extensions, but Struct.to_json(zarr_format=3) was still emitting an UnstableSpecificationWarning ("this data type does not have a Zarr V3 specification"). That message is now factually wrong for struct, and it forces every test touching structured arrays to carry a filterwarnings("ignore") marker.

This removes the warning for struct only.

Audit of all data types that emit v3_unstable_dtype_warning

I checked every call site against the published specs in zarr-extensions:

Written v3 name	Class	Spec in zarr-extensions?	Action
struct	Struct	✅ Yes — stable, name matches	Stop warning
structured	Structured	⚠️ Yes, but a read-only legacy alias — spec states implementations MUST NOT write it	Keep warning (write is discouraged)
variable_length_bytes	VariableLengthBytes	⚠️ Concept is spec'd, but under the name bytes, not variable_length_bytes	Keep (name mismatch — separate decision)
null_terminated_bytes	NullTerminatedBytes	❌ No spec	Keep
raw_bytes	RawBytes	❌ No spec (the r core type is raw bits, different)	Keep

So struct is the only one that cleanly qualifies; the other four warnings are intentionally left in place.

Changes

• Struct.to_json no longer calls v3_unstable_dtype_warning. The sibling Structured (legacy alias) and the three bytes data types are untouched.

Notes / follow-ups (out of scope here)

• variable_length_bytes is written under a name that doesn't match its spec (bytes). Resolving that is an interop/rename decision, separate from this warning cleanup.
• The ~25 filterwarnings("ignore::...UnstableSpecificationWarning") markers across the test suite are left as-is; many also cover the bytes/structured types that still warn. They can be pruned for struct-only tests in a follow-up.

Verification

• uv run pytest tests/test_dtype/ → 713 passed, 5 skipped.
• Confirmed at runtime: Struct no longer warns; Structured, null_terminated_bytes, raw_bytes, variable_length_bytes still do.
• test_unstable_dtype_warning (in test_dtype/test_npy/test_bytes.py) only asserts warnings for the three bytes types, so it is unaffected.

🤖 Generated with Claude Code

https://claude.ai/code/session_019rRWaxXfZ3ZmbiZYWZoDBP

[d-v-b]

self-merging this, as it's a safe QOL improvement.