Decouple SoftBreakMode from significantNewlines

[dereuromark]

Summary

• Removes automatic SoftBreakMode::Break when significantNewlines is enabled
• These two features are now independent, as they serve different purposes:
  • significantNewlines: markdown-like block interruption (parser behavior)
  • SoftBreakMode: visible line breaks (renderer behavior)

Breaking Change

Users relying on withSignificantNewlines() to render soft breaks as <br> need to explicitly set the mode:

// Before (implicit)
$converter = DjotConverter::withSignificantNewlines();

// After (explicit)
$converter = DjotConverter::withSignificantNewlines(
    softBreakMode: SoftBreakMode::Break,
);

See UPGRADE.md for full migration guide.

Rationale

The previous bundling was confusing because:

• The factory method name didn't suggest it also changed soft break rendering
• Users wanting markdown-style block interruption didn't necessarily want visible soft breaks (or vice versa)

Additionally, SoftBreakMode::Break is rarely needed in Djot because hard breaks are simple and visible:

Line one\
Line two

Compare to Markdown's awkward options:

• Two trailing spaces (invisible, easy to miss/delete)
• <br> HTML tag (verbose)

In Djot, if you want visible line breaks, you explicitly add \ - it's intentional and clear. This makes SoftBreakMode::Break a niche feature for specific use cases like importing content or poetry/lyrics where adding \ to every line would be tedious.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.21%. Comparing base (5b6b08f) to head (15ba0e6).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #141      +/-   ##
============================================
- Coverage     94.21%   94.21%   -0.01%     
+ Complexity     2570     2569       -1     
============================================
  Files            88       88              
  Lines          6847     6845       -2     
============================================
- Hits           6451     6449       -2     
  Misses          396      396              

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