## Why
The TUI renders supported inline math as Unicode but leaves display equations to ordinary Markdown rendering. Display fractions need spatial layout, and streamed equations must remain mutable while their closing delimiter is pending.
## What changed
- Render supported standalone `$$...$$` and `\[...\]` equations as Unicode, with stacked fractions and aligned baselines.
- Fall back to source for unsupported expressions, nested fractions, and spatial layouts that exceed the available width or occur inside lists or block quotes.
- Preview unfinished display math as wrapped source and hold it out of scrollback within the conversion budget, then render it when closed.
- Preserve display delimiter boundaries across incremental rendering and bound lookahead for rejected openers so shell `echo $$` does not retain the entire response.
## Testing
Add snapshots and regression tests for display layouts, narrow widths, literal fallback, streaming closure, resize reflow, raw source preservation, shell dollars, and bounded incremental rendering.
GitOrigin-RevId: ab54675f2e27a243f5d7165c76d9d7355f9fabd1
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