Fix footnote references crossing source newlines

[dereuromark]

Summary

• stop footnote-reference labels at LF or CR
• preserve same-line spaces and tabs exactly
• cover LF, CRLF, CR, multiline definition markers, warnings, and rendered output

Why

Footnote definitions are line-based, but inline references previously accepted source newlines. That produced identifiers that no valid definition could bind.

Fixes the djot-php behavior reported in jgm/djot.js#146.

Verification

• focused regression suite: 6 tests, 16 assertions
• full composer check: 2,766 tests, 18,386 assertions; code style passes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.41%. Comparing base (20073dc) to head (6b08341).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #267   +/-   ##
=========================================
  Coverage     92.40%   92.41%           
  Complexity     3673     3673           
=========================================
  Files           109      109           
  Lines         10421    10421           
=========================================
+ Hits           9630     9631    +1     
+ Misses          791      790    -1     

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