php-in-php: unlink() JIT via UnlinkJitHelper PHP — drop libc LLVM (#15471)

[PurHur]

Summary

• Route JIT unlink() through UnlinkJitHelper → VmFs::unlink() (same SSOT as VM, php-in-php: VmFsUnlink — VmFsUnlinkPure SSOT, drop libc unlink FFI (#1492) #12194)
• Replace ~18 LOC libc unlink(2) LLVM in JitUnlink.php with thin StringUnlink bridge (pattern from php-in-php: chmod() JIT via ChmodJitHelper PHP — drop libc chmod LLVM in JitChmod.php #15458 chmod)
• C runtime shrink: no new C; lookupFunction('unlink') removed from JIT lowering path

php-src reference

• ext/standard/filestat.c — php_unlink

PHP implementation

Path	Role
ext/standard/UnlinkJitHelper.php	invokeArgv() + warning on failure
lib/JIT/Builtin/StringUnlink.php	__phpc_jit_unlink ABI bridge
ext/standard/JitUnlink.php	Thin wrapper (was libc LLVM)

Verification

# Architectural — no libc unlink in JIT lowering
grep "lookupFunction('unlink')" ext/standard/JitUnlink.php
# (no output)

# Unit guard
./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter UnlinkRuntimeShrink'
# OK (4 tests, 11 assertions)

# Spine sync
php script/check-selfhost-spine-coverage-sync.php
# check-selfhost-spine-coverage-sync: OK

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

Closes #15471

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