Normalize navigation labels for block validity

[chubes4]

Summary

• Normalize HTML navigation labels before serializing core/navigation-link and core/navigation-submenu blocks.
• Strip block-level wrapper tags and empty aria-hidden icon elements from navigation labels, matching the existing button label normalization behavior.
• Add contract coverage for direct nav links, submenu labels, and nested nav links sourced from block-level label markup.

Matrix cluster summary

• Missing core/navigation representation: 20 findings across header/footer/sidebar/main nav landmarks.
• Navigation item count mismatch: 27 findings across ul/li/a, direct nav > a, footer/sidebar navs, and nested menus.
• Nav landmark under-representation: 12 findings where source nav landmarks exceed generated core navigation blocks.
• Item label/URL mismatch: 2 findings.
• Navigation block validity: 5 findings from block-level markup inside serialized link labels.
• Mobile drawer/runtime duplication also appears through runtime target diagnostics and existing dedupe coverage, but was not the smallest safe fix here.

Chosen fix

The clearest small, generic, high-confidence fix is navigation label normalization. It directly targets the wp_block_validity_navigation_block_level_link_markup history without fixture selectors and improves generated navigation validity for direct links, list menus, and nested submenu labels.

Verification

• composer test

Expected matrix impact

• Expected to clear or reduce the 5 navigation block validity findings with reason: Serialized link text contains block-level markup that WordPress static link blocks do not save as valid inline RichText.
• Not expected to materially change missing menu, item count mismatch, or landmark mismatch buckets; those need broader navigation recognition/projection work.

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Finding packet clustering, implementation, regression test drafting, and verification.