Refactor BodyChild traversal policy

[kiki830621]

Summary

• Add a shared BodyChildVisitor / BodyChildWalker with explicit recursion policy for table cells, nested tables, and content controls.
• Migrate DocumentWalker.walkAllParagraphs to the shared walker.
• Add policy coverage proving tables can be skipped while SDTs still recurse.

Notes

• This is the first low-risk adoption step for Refactor: extract BodyChildVisitor protocol to dedupe 5+ body-children walkers #28. Mutation-heavy walkers can migrate incrementally in follow-up PRs to avoid conflicts with active issue PRs.

Tests

• swift test --filter DocumentWalkerTests
• swift test

Refs #28