Stdlib: mail() sendmail_path popen transport (#3285)

[PurHur]

Summary

• Implement mail() sendmail delivery in PHP (VmMail::send) matching php-src ext/standard/mail.c php_mail(): popen(sendmail_path), RFC822 To/Subject/headers/body on stdin, accept EX_OK/EX_TEMPFAIL.
• Wire bin/vm.php -d sendmail_path=… / PHPT --INI-- via VmIni::applyStartupIniOverride → mirrored INI override (same pattern as browscap).
• Compliance: mock sendmail fixture + transport .phpt; keep “false when sendmail missing” registration case.
• Trust: remove duplicate testVmRegistersMbUcwordsOnForwardProfile that fatals PHPUnit on master.

php-src ref: ext/standard/mail.c (PHP_FUNCTION(mail), php_mail).
PHP impl: ext/standard/VmMail.php, ext/standard/mail.php.

Closes #3285

Batch next (claimed): #21432 headers array, #21433 mail.add_x_header, #21434 additional_params / force_extra_parameters.

Test plan

• php bin/vm.php -d sendmail_path=…/mock_sendmail.sh test/repro/issue_3285_mail_sendmail.php → true / OK
• vendor/bin/phpunit test/compliance/VMTest.php --filter 'mail_sendmail_transport|mail_registered' → OK (4)
• php script/bootstrap-inventory.php --check → OK 6163/6163
• CompilerVersionGateTest loads without fatal redeclaration

Verification transcript

true
OK
OK (4 tests, 4 assertions)
OK 6163/6163
gate_ok

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