feat: Add RTL (Right-to-Left) text support for Hebrew, Arabic, and bidirectional languages

[shaal]

Summary

This PR adds BiDi (Bidirectional) text support to Textual, enabling proper display of RTL (Right-to-Left) languages like Hebrew and Arabic in terminal applications.

Key Features

• Auto-detection: The text-direction: auto CSS property automatically detects RTL characters and applies the Unicode BiDi algorithm
• CSS property: New text-direction property with values auto, ltr, rtl
• Optional dependency: Uses python-bidi library (install with pip install textual[bidi])
• Graceful fallback: Works without python-bidi installed (RTL detection still works, just no reordering)

Changes

File	Description
_bidi.py	New core BiDi utilities module
content.py	Integrated BiDi processing in render pipeline
css/types.py	Added TextDirection type
css/constants.py	Added VALID_TEXT_DIRECTION
css/styles.py	Added text_direction CSS property
css/_styles_builder.py	Added CSS parser for text-direction
widgets/_text_area.py	Added text_direction reactive property
widgets/_markdown.py	Code blocks force LTR to preserve formatting
pyproject.toml	Added python-bidi optional dependency
tests/test_bidi.py	Comprehensive tests

Usage

CSS:

/* Auto-detect RTL text */
Static {
    text-direction: auto;
}

/* Force RTL */
Label {
    text-direction: rtl;
}

/* Force LTR (default for code) */
MarkdownFence {
    text-direction: ltr;
}

Python (TextArea):

text_area = TextArea(text="שלום עולם")
text_area.text_direction = "auto"  # or "rtl" or "ltr"

Test Examples

• Hebrew: שלום עולם (Hello World)
• Arabic: مرحبا بالعالم
• Mixed: Hello שלום World
• Numbers in RTL: מספר 123

Test plan

• Run pytest tests/test_bidi.py - All 22 tests pass (3 skipped when python-bidi not installed)
• Run pytest tests/test_content.py - All 74 tests pass
• Manual testing with Hebrew/Arabic text in Static, Label, TextArea widgets
• Verify code blocks in Markdown remain LTR

🤖 Generated with Claude Code

[willmcgugan]

I will need the following before I consider merging:

• Passing tests
• Snapshot tests covering new code paths
• Documentation and examples of the new styles in same format as we currently have for other styles
• A technical document describing the changes
• A commitment from you to support these changes for a while until bugs have been worked out

I gather this is AI generated. But I do hope you have an understanding of this update beyond what you prompted for...

https://textual.textualize.io/blog/2023/07/29/pull-requests-are-cake-or-puppies/

[yarons]

@willmcgugan It is a required feature because the text is reversed, I'll try to better look at it and see if there's something from this code that's usable for this use case.
I think using python-fribidi is a better approach than suggested, the overall directionality doesn't work anyway and it's not in the scope of this PR.