feat(sandbox): seccomp-notify DNS-pinned allowlist for Platform mode

Add kernel-level connect() interception using SECCOMP_RET_USER_NOTIF.
The supervisor intercepts network syscalls, reads the destination
sockaddr from the child's memory via /proc/pid/mem, evaluates it
against a DNS-pinned allowlist, and either performs the operation on
behalf of the child via pidfd_getfd() or denies it with EPERM.

Components:
- DnsPinnedAllowlist: resolve domains to IPs at sandbox creation,
  freeze for session lifetime to prevent DNS rebinding
- BPF filter with AUDIT_ARCH validation for connect/sendto/sendmsg/
  recvfrom/recvmsg/bind syscalls
- pidfd_open + pidfd_getfd for TOCTOU-safe on-behalf-of operations
- parse_sockaddr with correct endianness for IPv4/IPv6
- read_process_memory with read_exact for short-read safety

Signed-off-by: Ladislav Smola <lsmola@redhat.com>

Author: Ladas

crates/openshell-sandbox/src/lib.rs

@@ -23,6 +23,7 @@ pub mod procfs;
 mod provider_credentials;
 pub mod proxy;
 mod sandbox;
+pub mod seccomp_notify;
 mod secrets;
 mod skills;
 mod ssh;