feat: add ancillary data (control message) helpers for sendmsg/recvmsg

[seuros]

Closes #614

Add Unix-only safe wrappers for CMSG_* operations:

• cmsg_space(data_len) -> Option<usize>: compute control buffer size (returns None when data_len overflows c_uint, fixing a truncation path that would produce an undersized buffer)
• ControlMessage<'a>: a parsed ancillary data entry (level, type, data)
• ControlMessages<'a>: iterator over a received control buffer; walks via byte-offset arithmetic and ptr::read_unaligned so no aligned reference to cmsghdr is ever formed (avoids UB on 1-byte-aligned Vec buffers)
• ControlMessageEncoder<'a>: builder for outgoing control messages; rejects payloads exceeding c_uint::MAX before calling CMSG_SPACE/CMSG_LEN to prevent buffer overflow from silent truncation

Also add a Cross CI job to run tests under QEMU on i686, aarch64, and armv7 Linux, the target families where CMSG_ALIGN factor and cmsg_len width differ from x86_64.

Enables SCM_RIGHTS file-descriptor passing without depending on libc directly (tracked by plabayo/rama#781).

I can remove the Cross tests if needed.

[seuros]

The only remaining CI failure is FreeBSD (Cirrus CI) — this is an infrastructure flake, not a code issue:

error: failed to get `libc` as a dependency of package `socket2 v0.6.3`
Caused by: download of config.json failed
Caused by: failed to download from `https://index.crates.io/config.json`
Caused by: [60] SSL peer certificate or SSH remote key was not OK
  (SSL certificate OpenSSL verify result: unable to get local issuer certificate (20))

Cargo tries to update the crates.io index and hits an SSL cert verification failure in the Cirrus FreeBSD 14.3 VM. Unrelated to this PR! Verified manually on FreeBSD 16-Current where cargo build, cargo build --no-default-features, and all cmsg tests pass cleanly.