Add fuzz testing infrastructure

[dereuromark]

Summary

• Add nikic/php-fuzzer as dev dependency for automated fuzz testing
• Create fuzz testing targets for both normal and strict (warnings-enabled) modes
• Add djot syntax dictionary for guided fuzzing (contains common markup fragments)
• Add seed corpus with representative djot structures
• Add comprehensive EdgeCaseTest (82 tests) covering robustness scenarios:
  • Empty and whitespace inputs
  • Unclosed delimiters
  • Deep nesting
  • Long inputs
  • Special characters (unicode, control chars, emoji)
  • Escape sequences
  • HTML entities
  • Mixed syntax combinations
  • Boundary conditions

Usage

# Run fuzz testing
composer fuzz

# Run with warnings enabled
composer fuzz-strict

Test plan

• All existing tests pass (1298 tests)
• All new edge case tests pass (82 tests)
• Code style checks pass
• Fuzzer runs without crashes on initial test

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.72%. Comparing base (5085c8e) to head (ef45997).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #53      +/-   ##
============================================
+ Coverage     93.64%   93.72%   +0.08%     
- Complexity     1916     1928      +12     
============================================
  Files            74       74              
  Lines          5143     5177      +34     
============================================
+ Hits           4816     4852      +36     
+ Misses          327      325       -2     

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