bug(core): Context Packet fallback selectors can target the wrong element

[vamgan]

Summary

Fallback selectors emitted in Context Packets can fail to identify the selected element.

Root cause

packages/core/src/context.ts, buildElementSelector(), computes a document-global index among annotated elements of the same tag and emits :nth-of-type(index). CSS nth-of-type() is sibling-relative, not document-global.

For repeated tags under different parents, generated selectors can match nothing or the wrong element. The method also scans every same-tag element for each packet.

Proposed fix

• Prefer unique id and data-askable-id.
• Otherwise construct a parent-relative selector path using sibling-local indexes.
• Validate that the selector resolves uniquely to the original element before returning it.
• Avoid a document-wide same-tag scan.

Acceptance criteria

• Repeated tags under different parents resolve correctly
• Unannotated siblings are handled
• IDs and attribute values are safely escaped
• Duplicate data-askable-id values do not produce a falsely unique selector
• Shadow DOM behavior is documented/tested
• Packet-generation benchmark covers large documents