H1 headings are body content, not the title source of truth. Remove
useHeadingTitleSync hook and frontmatter-title-from-H1 logic in the
editor. Rust rename_note no longer modifies H1 in file content — only
updates the frontmatter title: field.
TitleField now uses optimistic UI: the new title displays immediately
after commit while the async file rename runs in background, preventing
UI freezes on slow filesystems or large vaults.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>