Restore submenus split across responsive navigation variants

[chubes4]

Summary

• reconcile duplicate responsive menu items when the visible variant is shallow and another variant owns the submenu tree
• recognize list-backed submenu wrappers structurally instead of requiring submenu or dropdown naming tokens
• emit one canonical core/navigation tree with native core/navigation-submenu children

Root cause

Responsive source sites can split navigation semantics across variants. The visible desktop menu on https://www.carajane.ca carries the top-level Portfolio item, while a duplicate responsive item with the same stable identity owns the Love, Newborn, Family, and Boudoir submenu. Blocks Engine selected the shallow visible variant and did not recognize the richer menu-wrap > ul structure as a submenu. Static Site Importer then correctly persisted that incomplete canonical markup.

Evidence

• Before: Portfolio persisted as a core/navigation-link with no URL and no children.
• After: Portfolio persists as core/navigation-submenu with four working local routes.
• Live WordPress 7.0.4 editor verification: zero invalid blocks across all 8 imported pages (712 blocks).
• Closed-page visual comparison against the unpatched import: 0 mismatched mobile pixels; 1.05% desktop mismatch localized to the now-functional submenu control. Full-page dimensions remain identical.
• Development package provenance: Static Site Importer f04c7de7ffb0, Blocks Engine dd78832d48e6.

The broader wrapper-heavy authoring model is separate and remains tracked by #868.

Testing

• composer test (278 parity fixtures plus canonical, unit, and packaging contracts)
• direct transform of the captured Cara Jane homepage: 1 navigation, 1 submenu, 8 child links, block validity pass
• Studio PR 3952 artifact re-import using current Static Site Importer main and this branch
• browser verification of submenu hover and all four destinations
• browser editor verification across all imported pages

AI assistance

• Model/tool: OpenAI GPT-5.6 Sol via OpenCode
• Used for: Root-cause analysis, implementation, regression coverage, package construction, live Studio import verification, browser interaction checks, and visual comparison.
• Chris Huber reviewed and remains responsible for the change.