Typed TableInfo / Paragraph.table_info

Surfaced during [pprose](https://github.com/jlevy/practical-prose) structural-metrics work after the chopdiff 0.3.1 upgrade. Not blocking — pprose has a source-text workaround — but landing this lets pprose drop it.

**Need:** Per table block, row / cell / column counts and per-column alignments.

**Proposed API** (valid when `block_type` is `table`):

```python
Paragraph.table_info -> TableInfo | None

class TableInfo:
rows: int
cols: int
cells: int
alignments: list[Literal["left", "center", "right", None]]
```

**pprose workaround today:** Count rows/cells from the `table` block's source lines.

_Tracked in pprose as `pp-eaa2`._