php-in-php: delete EnvironMirrorRuntimeUserScriptCstr — helper-runtime + libc native mirror (#19157)

[PurHur]

Summary

• Delete EnvironMirrorRuntimeUserScriptCstr.php (~182 LOC libc environ LLVM defer path) and route all user-script/embed paths through JitVmHelperLink + EnvironMirrorNativeJitHelper.
• Init-safe mirror SSOT: VmEnvEnvironNative::mirrorIntoNativeHashtable() → phpc_native_environ_mirror_into_ht → shared EnvironLibcWalkJit (libc environ walk into native __hashtable__*).
• Prelink ext/standard/EnvironMirrorNativeJitHelper.php helper-runtime unit for cold-start user-script AOT.

C runtime shrink

Removed	Added
lib/JIT/Builtin/EnvironMirrorRuntimeUserScriptCstr.php (182 LOC dedicated defer LLVM)	EnvironLibcWalkJit.php (shared JIT helper) + thin phpc_native_environ_mirror_into_ht internal

Net: dedicated user-script-only defer branch deleted; libc walk consolidated behind PHP internal callable from compiled helper.

php-src reference

• sapi/cli/php_cli.c — copy environ into superglobals on CLI startup

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php vendor/bin/phpunit test/unit/EnvironMirrorRuntimeShrinkTest.php'
# PHPUnit 9.6.21 — OK (6 tests, 16 assertions)

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && echo "<?php echo \"hi\";" > /tmp/hi.php && PHP_COMPILER_AOT_USER_SCRIPT=1 php bin/compile.php -o /tmp/hi_bin /tmp/hi.php && /tmp/hi_bin; echo exit=$?'
# phpc build: helper-runtime cache hit (1 helpers, core=b868f1cb704c8fe544e6)
# hi
# exit=0

make bootstrap-selfhost-vm-driver-execute-probe
# bootstrap-selfhost-vm-driver-execute-probe: OK

Closes #19157

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