test: add nested empty-object list item fixtures

[montanaflynn]

Summary

• Add encode/decode fixtures for nested expanded arrays containing empty object list items.
• Confirm recursive empty-object list items encode as a bare - marker with no trailing space.
• Record the fixture addition in the changelog.

Rationale

The spec already defines this behavior:

• §9.4 says expanded list items may use a bare - marker for empty object list items.
• §10 says an empty object list item is a single - at the list-item indentation level.
• §12 says encoders MUST NOT emit trailing spaces.

The reference implementation follows that rule recursively. encodeObjectAsListItemLines emits LIST_ITEM_MARKER for empty objects, where LIST_ITEM_MARKER is "-" and LIST_ITEM_PREFIX is "- " for non-empty list items:

• https://github.com/toon-format/toon/blob/main/packages/toon/src/encode/encoders.ts#L287-L293
• https://github.com/toon-format/toon/blob/main/packages/toon/src/constants.ts#L3-L4

I verified the current reference implementation directly:

items[2]:
  - [1]:
    -
  - [2]:
    - x
    -

decodes back to:

{"items":[[{}],["x",{}]]}

Coverage gap

The current fixtures cover empty object list items at the outer list level, for example:

items[3]:
  - first
  - second
  -

They also cover arrays of empty objects as outer list items. What was missing is the recursive nested-array case:

{"items":[[{}],["x",{}]]}

That gap lets an encoder special-case empty objects in top-level expanded arrays while still emitting - with a trailing space for empty objects inside nested expanded arrays. This PR adds encode and decode coverage for that recursive case.

Validation

• node --input-type=module -e "JSON.parse(...)" for modified fixture JSON
• npx --yes ajv-cli validate -s tests/fixtures.schema.json -d "tests/fixtures/**/*.json"
• npm run lint
• node --experimental-strip-types --input-type=module -e "import { encode, decode } from './packages/toon/src/index.ts'; ..." in toon-format/toon
• zig build test --summary all in LatentEvals/toon-zig