feat(pvisor): separate isolation policies and support static musl builds

[lizhicui]

Summary

• separate OverlayNet policy, filesystem access restrictions (--filesystem sandbox), and workspace staging (--stage), so network-only runs can write benchmark and harness directories with normal host permissions
• support static x86_64 Linux musl VM builds with embedded libkrun kernel bundles and an extraction helper; report unsupported VM platforms explicitly
• fix Alpine/BusyBox applet execution by preserving the original command name, and resolve musl statx, time type, and fuser cross-compilation compatibility issues
• update English and Chinese documentation for the independent isolation settings; preserve command failures and exit codes
• align isolation tests, the filesystem-isolation example, and CI helpers with the explicit --filesystem sandbox opt-in; the same commit fixes a bash 5.2 heredoc parse bug in the nightly installer and widens a replay watchdog timing margin for loaded macOS runners

Validation

• cargo fmt --all -- --check, cargo check --locked -p persisting-pvisor, and the x86_64 musl release cross-build passed during development; git diff --check passed before publication
• remote regression covered Alpine/musl, AlmaLinux/glibc, and namespace-limited Linux environments: filesystem writes, BusyBox commands, exit-code propagation, and proxy allow/deny policies
• deny-all blocked direct traffic where network namespaces were available; environments without namespace support fell back with a warning, while strict mode rejected the unavailable isolation guarantee (direct sockets remain ambient in the fallback)
• VM startup checks covered missing KVM diagnostics; a complete VM workload regression requires a host with KVM and was not performed on the available test hosts
• after the alignment fixes, every GitHub Actions check passes on the PR head, including the full macOS workspace suite on the hosted runner