Delivered comments report incorrect positions within their sibling group

Problem/Motivation

Comments loaded on demand expose their position within the delivered slice instead of the full accessible sibling group. For the 50th of 100 root comments, initial full-index rendering reports position 50 of 100, while individual-comment and delivery responses report 1 of 1. Newly inserted rows retain the incorrect attributes.

Steps to reproduce

1. Create 100 accessible root comments and use progressive loading.
2. Load the 50th comment through a bounded delivery or individual-comment request.
3. Inspect the inserted row's aria-setsize and aria-posinset; compare with its full sibling group.

Proposed resolution

Report full accessible sibling counts and positions without restoring full sibling-entity hydration. Preserve the separate flat-result semantics used by filtering.

Remaining tasks

• Test initial rendering and rows inserted by delivery, branch, context, and live refresh.
• Cover roots, replies, descending order, and inaccessible siblings.
• Verify attributes in the resulting browser tree, not only a render array.

User interface changes

Assistive technology receives correct sibling-position information after on-demand loading.

API changes

Sibling metadata may need to accompany bounded indexes or delivery state; document the chosen contract.

Data model changes

None anticipated.