* chore(zarr-metadata): note the sdist allowlist, and let misc entries speak

#4248 gave this package's sdist an explicit allowlist and merged without a
news fragment, so a user-visible packaging change was about to miss the
0.5.0 notes. Add one.

Filing it as `misc` exposed that towncrier's built-in `misc` type sets
`showcontent = false`: the entry would render as a bare PR link, which tells
a reader nothing. Restate all five types the `changes/README.md` menu offers
— declaring any type replaces the built-in set — as the defaults verbatim
except for `misc`, which now shows its content. A change worth a release
note is worth a sentence, whatever its category.

Assisted-by: ClaudeCode:claude-opus-5

* chore(zarr-metadata): build 0.5.0 changelog

Consume the pending news fragments — #4232's constant-naming-grammar removal
note, #4264's note widening `JSONValue`'s array arm to the covariant
`Sequence`, and #4248's sdist allowlist — into CHANGELOG.md via towncrier
for the zarr_metadata-v0.5.0 release.

Minor, not patch: the `JSONValue` widening changes a published type's
meaning for every consumer that annotates against it, and #4232 removes the
old version-last constant spellings outright.

Assisted-by: ClaudeCode:claude-opus-5