[codex] Add changed clip ID merge mode

[explicit09]

Summary

Implements issue #3 by adding reusable vedit-core support for changed clip IDs and a merge path that uses clip-ID overlap instead of track-level conflict semantics.

Changes

• Parse stable clip IDs from OTIO clip fields/metadata into the core model.
• Add reusable changed-clip-ID APIs for timeline comparison and Repo::changed_clip_ids(ref) relative to the first parent.
• Add Repo::merge_changed_clip_ids(source, target, author, message) with structured clean/conflict payloads.
• Allow same-track non-overlapping clip-ID merges as overlay updates.
• Return typed overlap conflicts without committing.

Validation

• cargo fmt
• cargo test
• cargo clippy --all-targets -- -D warnings

Closes #3