Stdlib: readlink() VM + LLVM JIT/AOT via libc readlink(2)

[PurHur]

Problem

Symlink targets are read with readlink() in deploy and asset tooling; the compiler has is_link() but not readlink().

Goal

• readlink(string $path): string|false
• VM via VmFs; JIT/AOT via libc readlink(2) (no PHP internal_* wrappers on the LLVM path)

Tests

• test/compliance/cases/stdlib/readlink*.phpt
• test/fixtures/aot/cases/readlink.phpt
• PHPUnit smoke in test/unit/ReadlinkBuiltinTest.php

Verification

./script/ci-local.sh --filter readlink in Docker (php-compiler:22.04-dev).