Property-based tests + CONTRIBUTING.md

[mmucklo]

Summary

Adds property-based (randomized invariant) testing and a contributor guide. Stacks on top of PR #54 (test-strengthening).

Property-based tests

10 randomized invariants, 200 iterations each, using native PHPUnit + mt_rand (no external dependency). Deterministic via SEED envvar for failure reproduction.

Property	Invariant
No-crash (parseSingle)	Every byte string yields a ParsedEmailAddress; never an exception
No-crash (parseMultiple)	Same for multi-address mode
Determinism	parse(x) == parse(x) for all x
Reason + code consistency	invalid=true ⇔ both invalidReason and invalidReasonCode non-null
Severity	Every invalid address has a non-null ValidationSeverity
Stringable	(string) $parsed === simpleAddress when valid, '' when invalid
toArray round-trip	parseSingle(x)->toArray() === parse(x, false) for all x
Valid round-trip	Synthesized atext@label.tld parses; reparsing simpleAddress yields the same simpleAddress
All presets	All four factory presets (rfc5321/rfc6531/rfc5322/rfc2822) never crash on arbitrary input

Finding: no bugs or crashes across 2000+ random inputs including arbitrary byte sequences (NUL, control chars, invalid UTF-8). The parser is robust to malformed input.

CONTRIBUTING.md

Developer guide covering: dev setup, all 7 composer scripts, adding test cases via testspec.yml, code-style rules, PHPStan level 8, PR conventions, RFC citation requirements, issue template.

Numbers

• 84 tests / 3,279 assertions (up from 75 / 589 in PR Strengthen tests: MSI 74% → 80%, fix quoted-pair bug #54)
• CI green: cs:check, PHPStan level 8, all tests

Test plan

• composer ci passes
• Property tests pass with default (time-based) seed
• Deterministic reproduction works: SEED=12345 composer test produces identical results
• CONTRIBUTING.md reviewed for accuracy against current composer scripts

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.21%. Comparing base (97ad7ac) to head (c059328).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #55      +/-   ##
============================================
+ Coverage     90.88%   93.21%   +2.33%     
  Complexity      382      382              
============================================
  Files             6        6              
  Lines           987      987              
============================================
+ Hits            897      920      +23     
+ Misses           90       67      -23     

see 1 file with indirect coverage changes

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