wires user configured `FontFallbacks` into the cosmic text path. the
chain is resolved at font load time and stored on each `LoadedFont`.
`layout_line` splits each `FontRun` into spans by codepoint coverage and
emits one `Attrs` per slot so cosmic text shapes each span with the
correct face. inheriting codepoints (marks, zwj, zwnj, variation
selectors) stick to the current span so emoji zwj sequences and
combining marks are not torn across faces.

Closes #17254

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] Performance impact has been considered and is acceptable
- [x] Tests cover the new/changed behavior
- [] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)

Release Notes:

- added support for `buffer_font_fallbacks` on linux