Trailing bottom margin on last block (paragraph/list) causes extra spacing

[zollipaul]

Problem
The last rendered block always keeps a bottom margin (e.g., 4px), so StreamdownRN leaves extra space at the end even when the container has no padding. This is especially visible with short content or list-only messages.

Where it comes from

• Paragraphs/headings apply marginBottom: theme.spacing.block in src/themes/index.ts.
• Several block containers also use marginBottom: theme.spacing.block in src/themes/index.ts.
• List rows hard-code marginBottom: 4 in src/renderers/ASTRenderer.tsx.

Steps to reproduce

1. Render a single paragraph or list with StreamdownRN.
2. Set theme.spacing.block to a small value (e.g., 4).
3. Observe extra spacing after the last block.

Example:

<StreamdownRN
  theme={{
    colors: { /* … */ },
    fonts: { /* … */ },
    spacing: { block: 4, inline: 4, indent: 16 }
  }}
  isComplete
>
  {'Hello world'}
</StreamdownRN>

Expected
No trailing bottom margin on the final rendered block.

Actual
A bottom margin equal to theme.spacing.block (and 4px for last list item) remains.

Environment

• streamdown-rn: ^0.2.1
• React Native / Expo (app-side)

Suggested fix
Add a way to drop spacing on the final block, e.g.: Pass isLastBlock from StreamdownRN to StableBlock / ActiveBlock. In ASTRenderer, conditionally set marginBottom: 0 when isLastBlock is true. Also set list row margin to index < last ? 4 : 0 in renderList.