Fix direct position assignment for embedded documents

[dblock]

Summary

• Make direct embedded position assignment shift siblings on child or parent save, clamp bounds, and move nil positions to the bottom.
• Preserve all existing movement APIs and their precedence; this is a scoped bugfix rather than the experimental rewrite in EXPERIMENTAL: Remove move_to and helper methods in favor of using position field directly. #88.
• Incorporate @johnnyshields’s regression tests from Failing specs for embedded doc position updating #87 and extend them with repeated saves, single-child lists, and explicit movement precedence.
• Document the behavior change and migration guidance in README and UPGRADING.md.

Supersedes #87 and #88, with credit to @johnnyshields for the reproductions and original investigation.

Prepared by an AI on behalf of @dblock.