AOT: deferred header() flush uses write(2) before body (#634)

[PurHur]

Summary

• Fix AOT CGI header ordering for header() + echo json_encode(...) web fixtures: __phpc_response_headers_flush now emits Status/header lines with write(2) instead of printf(3), so deferred headers are not stuck in the stdio buffer while body output uses direct write(2).
• Gate CGI stdout emission on getenv('REQUEST_METHOD') (CLI headers_list cases stay quiet).
• Do not mark the pending-header queue flushed when a flush pass wrote nothing; invalidate the LLVM header mirror on header_remove() so removed lines are not replayed.

php-src reference: ext/standard/head.c (sapi header emit before body).

Closes #634

Test plan

./script/aot-smoke.sh
# aot-smoke: 8 passed, 0 failed

./script/phpunit.sh --filter 'json_encode_api|echo_header_before_body|class_method_json_api|header_remove|headers_list'
# OK (8 tests, 35 assertions) — primary json/header ordering cases green

Not covered: full AotTest header filter (some cases like setcookie_headers_list segfault on master too), differential sweep, self-host gates.

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