Fix examples web smoke: ServeCompileCache constant mutation (#12040)

[PurHur]

Summary

• Root cause: ServeCompileCache reuses compiled Block instances across phpc serve requests, but Block::getFrame() placed compile-time constants into frame scope by reference. Builtins such as header() coerce arguments in place, so the cached literal for Content-Type: … became "" on the second request.
• Symptom: DevServer::respond() wrote an empty extra header line (\r\n only), terminating HTTP headers early; Content-Length no longer matched the body and curl truncated responses (002-StaticWeb GET / missing World).
• Fix: clone constant Variable cells per run via scopeConstantVariable(); ignore blank header() / extraHeaders lines defensively.

PHP implementation: lib/Block.php (scope setup), lib/Web/ResponseContext.php, lib/Web/DevServer.php.

Verification

./script/docker-exec.sh -- bash -lc './script/php-local.sh vendor/bin/phpunit test/unit/Web/ServeCompileCacheConstantsTest.php --no-configuration'
# OK (1 test, 6 assertions)

./script/docker-exec.sh -- bash -lc './script/examples-web-smoke.sh'
# examples-web-smoke: ok

./script/docker-exec.sh -- bash -lc './script/release-readiness.sh --full --json'  # spot-check examples-web-smoke gate

Closes #12040

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