Expose cross-platform descriptor byte-view I/O

[coenttb]

Summary

Freeze the platform producers whose existing typealias/product composition exposes canonical synchronous descriptor byte-view operations through Kernel:

• POSIX policy 006ba5e9a813c04896a40b8e0ed4435807acfd05, pinning ISO 9945 a877b6d48cd461c50b909821c0b2b1d0898176b2.
• Win32 producer 2b2e6e1055961c6922538dbb7eb74531eb72878c, re-exported through the existing swift-windows chain.

This exposes:

• Kernel.IO.Read.read(_:into: inout Swift.OutputSpan<Byte>) throws(Kernel.IO.Read.Error) -> Int
• Kernel.IO.Write.write(_:from: borrowing Swift.Span<Byte>) throws(Kernel.IO.Write.Error) -> Int

No duplicate unifier forwarding overload is added because Kernel.IO is already a platform typealias; redeclaring the same signature would create competing ownership. POSIX retains EINTR retry and unchanged would-block propagation. Win32 retains native ReadFile/WriteFile EOF, partial-completion, and typed-error semantics. OutputSpan frontier management remains in the deepest platform producer.

Issue: #14
Goal: swift-institute/.github#87

Static controls

• Public repository, guarded main 1cbcc9b9fdbe977d2c219ffcaca7a872751fefc5.
• Head f12f7def2eb02bd910d7e5e8d72160216f92b078 changes only Package.swift.
• Existing swift-windows Windows.Kernel.IO = Windows.32.Kernel.IO alias and Windows Kernel re-export were inspected at 41e619315c812bf3c24e358fe675ba7d2e9718a8; no wrapper is required.
• Commit/diff scanned for machine paths, private internals, credentials, and control-plane secrets; none present.
• Worktree clean after the commit.

Verification status

Explicitly UNVERIFIED. Under the source-only moratorium, no build, test, resolve, update, clean, run, compose, verify, typecheck, lint, compiler probe, CI read, or Package.resolved operation was performed.

[coenttb]

Semantic owner link (swift-institute/.github#591): this open PR implements swift-foundations/swift-kernel#14. Exact PR head at adjudication: f12f7de.