Preserve responsive counterpart provenance for captured desktop/mobile documents

## Problem

A fresh capture of https://www.biajmsb.ca/ produced editable desktop and mobile document variants. The homepage hero and event heading each occur twice in Gutenberg, but their emitted blocks have no be-responsive-counterpart-* class. Editing one variant and saving updates only that variant.

Blocks Engine deliberately pairs only blocks with stable provenance tokens, not equal text, so it correctly emits no responsive-counterpart editor module for this document.

## Reproduction

1. Capture the source with the current DLA document flow.
2. Import the artifact through the current SSI package and Blocks Engine.
3. In Gutenberg, edit the hero heading Explore the Future of Business Through Artificial Intelligence.
4. Save and reload. The desktop/mobile counterpart remains unchanged.

## Expected

DLA should preserve or declare stable cross-variant source identity for structurally equivalent captured elements so the platform-neutral Blocks Engine responsive-counterpart contract can emit its bounded editor mirror module.

## Evidence

The paired heading blocks render with independent classes and no counterpart token. The browser proof observed two matching editable blocks before edit and one persisted marker after reload.

AI assistance disclosure: OpenAI gpt-5.6-terra using OpenCode was used to reproduce the import, inspect emitted editor metadata, and draft this issue.

Start by tracing the current DLA document capture flow through the SSI package into Blocks Engine, then inspect the emitted Gutenberg block metadata for the duplicated hero and event headings. Determine where stable cross-variant source identity is lost or can be declared. Done means the imported variants emit bounded responsive-counterpart metadata and editing one variant persists the corresponding change to its counterpart.

Written by the indexing model from the issue text.