The `anydoc` converter's failure to distinguish between visible and hidden content in XLSX files, leading to hidden rows and columns being treated as visible in both `to_document()` and `to_markdown()` outputs. This is a data fidelity issue regarding content visibility.

## Summary

XLSX cells in hidden rows and columns are returned by `to_document()` and rendered by `to_markdown()` exactly like visible cells, with no visibility metadata or other indication that they are hidden in the source workbook.

This can make calculation/template content appear user-visible and authoritative in LLM input even though it is not visible when the workbook is opened normally.

## Reproduction

Tested with `firecrawl-anydoc==0.1.2` and `openpyxl==3.1.5`.

```python
from pathlib import Path

import anydoc
from openpyxl import Workbook

path = Path("hidden-content.xlsx")

workbook = Workbook()
sheet = workbook.active
sheet.title = "Visibility Example"
sheet["A1"] = "Visible row"
sheet["A2"] = "Hidden row"
sheet.row_dimensions[2].hidden = True
sheet["B1"] = "Hidden column"
sheet.column_dimensions["B"].hidden = True
workbook.save(path)

print(anydoc.to_markdown(path))

document = anydoc.to_document(path.read_bytes())
table = next(block.table for block in document.blocks if block.table is not None)
print([name for name in dir(table) if not name.startswith("_")])
```

## Observed

```text
| | |
| --- | --- |
| Visible row | Hidden column |
| Hidden row | |

['grid', 'header_rows', 'kind']
```

The hidden row and hidden column are indistinguishable from visible content in both outputs.

## Expected

Visibility should not be silently flattened away.

For Markdown, a reasonable default would be to omit hidden rows and columns, with an option to include them when d...