PPTX writer: keep tables and surrounding text on same slide

[ccb]

Summary

• Fix for PPTX writer where bullet points before and after a table would be split across multiple slides or rendered in incorrect order
• Tables and surrounding text now stay together on the same slide with proper vertical positioning

Changes

1. Presentation.hs: Modified splitBlocks' to continue accumulating content after tables instead of immediately splitting to a new slide

2. Presentation.hs: When a slide has a table with surrounding text, use ContentSlide (single column) instead of ContentWithCaptionSlide (two columns) to preserve content order

3. Output.hs: Added shapesToElementsStacked function to properly position multiple shapes (tables + text) vertically within a content area. Space is allocated proportionally based on content size (paragraphs and table rows)

Test plan

• All 3836 existing tests pass
• Added new test case table-with-surrounding-content
• Manually tested with various markdown files containing tables with surrounding bullet points

Example markdown that previously had issues:

## Slide Title

- Bullet before table
- Another bullet

| Col A | Col B |
|-------|-------|
| Val 1 | Val 2 |

- Bullet after table

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