Fix PHP 8.6 mb_regex_encoding deprecation in domain validation (#57)

[mmucklo]

Closes #57.

Problem

validateDomainName() used mb_regex_encoding() + mb_split() to split the domain into labels. Both emit E_DEPRECATED under PHP 8.6 (the underlying oniguruma library is no longer maintained):

Function mb_regex_encoding() is deprecated since 8.6, because the underlying library is no longer maintained

Fix

At that point the domain is already pure ASCII — it has been through idn_to_ascii() via normalizeDomainAscii() — so the multibyte machinery buys nothing:

• mb_split('\.', $domain) → explode('.', $domain)
• mb_substr($part, …, $encoding) hyphen-boundary checks → substr()
• dropped the now-unused $encoding parameter — the sole caller (Parse.php) never passed it, so it was always the 'UTF-8' default

Behavior is unchanged. Two now-stale psalm-baseline.xml entries (the removed mb_regex_encoding calls) are cleaned up.

Tests

Filled coverage gaps around the touched code and the MB/encoding paths it sits in — each assertion captured from the real parser output, not guessed:

Domain-label branches (testspec.yml)

• trailing-hyphen label (baddomain-.com) — the substr($part, -1) half of the boundary check had no test
• invalid-char label (bad_domain.com) — the per-label preg_match branch
• over-255-octet domain (domain_too_long)

Encoding / MB (ParseTest.php)

• non-UTF-8 tokenizer (ISO-8859-1, single-byte) and variable-width (Shift-JIS, 2-byte kanji) — proves mb_substr indexes by character, not byte
• malformed-UTF-8 rejection (InvalidUtf8Encoding)
• PunycodeConversionFailed and LocalPartCannotBeNormalized — previously referenced only in the severity-enum test, never triggered functionally
• RFC 5321 §4.5.3.1 length boundaries — accept-at-max / reject-one-over for local part (64), domain label (63), and whole address (254)
• mismatched caller-encoding domain fails gracefully (no crash)

Also documented in-source that the empty-label branch in validateDomainName is unreachable (the state machine rejects consecutive/edge dots first).

Verification

All green: composer test (91 tests, 3325 assertions), composer stan (PHPStan level 8), composer psalm, composer cs:check.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.06%. Comparing base (7fd7790) to head (ca0a0f8).

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #58      +/-   ##
============================================
+ Coverage     91.99%   93.06%   +1.07%     
+ Complexity      382      380       -2     
============================================
  Files             6        6              
  Lines           987      981       -6     
============================================
+ Hits            908      913       +5     
+ Misses           79       68      -11     

Files with missing lines	Coverage Δ
src/Parse.php	90.35% <100.00%> (+1.51%)	⬆️

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