Add Creole-style |= table header syntax

[dereuromark]

Summary

This PR adds extended table features to djot-php, providing more flexible table authoring options while maintaining full backward compatibility with standard djot syntax.

Features

1. Creole-style Header Cells (|=)

Mark individual cells as headers using the |= prefix (must be directly attached to pipe):

|= Name |= Age |
| Alice | 28   |
| Bob   | 34   |

Renders as:

<table>
<tr><th>Name</th><th>Age</th></tr>
<tr><td>Alice</td><td>28</td></tr>
<tr><td>Bob</td><td>34</td></tr>
</table>

2. Cell Alignment Markers

Alignment markers must be directly attached to |= (no space):

Syntax	Alignment
|=< text	Left
|=> text	Right
|=~ text	Center

Example:

|=< Left |=~ Center |=> Right |
| A      | B        | C       |

3. Colspan Support

Empty cells merge with the previous cell:

|= Name       ||= Details |
| First | Last | Age      |

The Name header spans 2 columns.

4. Rowspan Support

Use |^ (directly attached) to continue from the cell above:

|= Category |= Item  |
| Fruit     | Apple  |
|^          | Banana |
|^          | Orange |

The Fruit cell spans 3 rows.

5. Header Rowspan

Combine header and rowspan with |=^:

|= Group |= A |= B |
|=^      | 1  | 2  |
|=^      | 3  | 4  |

Compatibility Notes

Important: All special markers must be directly attached to the pipe character:

Input	Interpretation
|= text	Header cell with content "text"
| = text	Regular cell with content "= text"
|^	Rowspan marker
| ^	Regular cell with content "^"
|=<	Left-aligned header
|= <	Header with content "< ..."

This prevents false positives when users want literal =, ^, <, >, or ~ characters in their cells.

Additional compatibility:

• Traditional separator row syntax (|---|) continues to work as expected
• Mixed usage is supported: separator rows and |= syntax can coexist

Test Coverage

• All new features have comprehensive test coverage
• Tests verify marker attachment requirement (spaces prevent special interpretation)
• Backward compatibility verified with existing table tests

RFC Status

This extends beyond the standard djot specification. The syntax choices prioritize:

• Familiarity (Creole/MediaWiki-style |= for headers)
• Simplicity (single-character markers)
• Safety (markers must be attached to avoid false positives with literal content)

Future Considerations

Per-cell alignment for data cells (e.g., |< left | or |> right |) is not included in this PR. This would require a different syntax to avoid conflicts with literal < and > characters which are common in table data (comparisons like < 5, > 10, math expressions, etc.). This could be revisited in a future PR if there's demand, e.g.

|:< to left side |:> to right side |:~ centered |

Both mixed

If both are defined, the header would only align itself, and the meta row would define for the cells below:

|= Name  |=< Age |
|:------|----:|
| Alice | 28  |

Related

• Discussion: Simpler table markup? jgm/djot#354

TBD

• Include row/col spans using | and ^ ?
• Multi-line Cells using \ ?

This is also a more intensive change of the specs, while still being fully compliant, this does make documents that use this not working with other parsers.
So we would need a clear sign-off from the spec author(s) IMO before we go down this road - or it must be a clear feature flag.

[codecov]

Codecov Report

❌ Patch coverage is 97.80220% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.36%. Comparing base (5bb7d67) to head (56cc772).
⚠️ Report is 27 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Node/Block/TableCell.php	75.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master       #8      +/-   ##
============================================
+ Coverage     93.28%   93.36%   +0.08%     
- Complexity     1672     1695      +23     
============================================
  Files            59       59              
  Lines          4347     4434      +87     
============================================
+ Hits           4055     4140      +85     
- Misses          292      294       +2     

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