Avoid allocating the comma tail twice

[iliaal]

Summary

• Join the comma-separated given-name tail once and share that string between the reentrant-tail stack and the protected split hook.
• Remove a redundant string allocation without changing parsing decisions or public/protected signatures.
• Add a regression proving that a subclass can mutate its by-value hook argument without corrupting the cached tail or original source.

Measured benefit

On PHP 8.4.24, an exactly 1 MiB valid three-segment input (Smith, + 1,048,565 lowercase a characters + , MD) uses 1,052,672 fewer live bytes at hook entry and 1,052,672 fewer peak allocated bytes during parsing. Five before/after samples were identical. This saves one joined-tail string when there are multiple tail segments; single-element tails already share their string. No broad throughput improvement is claimed.

Validation

• PHP 8.4.24 / PHPUnit 12.5.29: 1,198 tests and 3,107 assertions passed with the 256 MiB test limit.
• PHPStan level 9 and PHP 8.5 compatibility analysis passed (local analysis memory limit 512 MiB).
• PHP CS Fixer dry run, 69 PHP syntax checks, Composer audit, baseline/README/deprecation guards, and diff checks passed.
• Independent review: 273 complete corpus outputs matched master; copy-on-write, nested success/failure, mismatched direct-hook calls, exception cleanup, and parser reuse checks passed.
• Verified all 84 remote file paths, modes, and blob hashes match the tested local tree before opening this PR; recursive tree was not truncated.
• Exact-head CI passed all five jobs: PHP 8.3 / 8.4 / 8.5 tests (including Composer audit), static analysis (including the PHP 8.5 compatibility gate), and code style. No jobs or steps were skipped.