Returns (name, AttachmentRef) pairs so callers can iterate an entry's
attachments together with the per-entry filename. Previously, callers
either had to know the names ahead of time and use attachment_by_name,
or iterate via attachments() and lose the name (AttachmentRef does not
expose the per-entry name key).

The motivating use case: serializing an entry's attachments to a list
of (name, data) pairs without a second lookup pass. Existing alternative
required serializing the whole Entry via the serialization feature and
extracting attachment names from JSON, which pulls in chrono/serde_json
and rebuilds the entire entry payload as a workaround.

Closes #314

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>