Preserve source navigation list gaps

[chubes4]

Summary

• Preserve the resolved gap from a direct navigation list child when the outer navigation does not define blockGap.
• Keep an explicit outer navigation gap authoritative.
• Add deterministic contract coverage for zero-gap projection and outer-gap precedence.

Closes #617
Relates to Automattic/static-site-importer#489

How to test

1. Run composer --working-dir=php-transformer test. Confirm all unit and contract checks, 235 parity fixtures, and package-install proof pass.
2. Review the navigation assertions in php-transformer/tests/contract/run.php. Confirm a direct list gap of 0 is projected and an explicit outer navigation gap retains precedence.
3. Run git diff --check origin/trunk...HEAD. Confirm it exits successfully with no whitespace errors.

Backwards compatibility

Backwards compatible for callers. Existing outer navigation blockGap remains authoritative; output changes only when a direct list child supplies spacing that was previously dropped.

Evidence bounds

• Change kind: Runtime fix
• Discriminator: A direct navigation list child has an explicit computed gap while the outer navigation has no mapped blockGap.
• Nearby contract preserved: Explicit outer navigation blockGap retains precedence.
• Why this is bounded: The change reads only the direct list child used as core/navigation content and only supplies blockGap when the outer navigation did not already define one.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode with openai/gpt-5.6-sol
• Used for: Investigated the visual parity defect and drafted the implementation and tests; Chris reviewed and owns the change.