fix(oci): render arm64 platform description without redundant v8

[Halvanhelv]

Summary

Platform.description renders the same arm64 platform two different ways
depending on how the value was constructed:

Platform(arch: "arm64", os: "linux", variant: nil).description  // "linux/arm64"
Platform(arch: "arm64", os: "linux", variant: "v8").description // "linux/arm64/v8"

These are the same platform — ==, hash(into:), and Set membership
already treat an arm64 nil variant as equivalent to "v8" — yet they
serialize differently, so a single platform drifts between linux/arm64 and
linux/arm64/v8 across stages of one build (apple/container#1542).

Relation to #764

#764 fixed the Hashable side of this: it stopped hash(into:) from using
description and canonicalized arm64 nil → v8 in the hash. That worked
around the inconsistent description but did not fix it — its own summary
names description (linux/arm64 vs linux/arm64/v8) as the root cause.
This PR fixes that remaining gap at the source.

Change

Omit the redundant v8 variant for arm64 when rendering description, so
equal arm64 platforms always describe as linux/arm64 — matching how Docker
and containerd display the platform. Other variants (arm/v7) and
architectures (amd64) are unaffected.

Only the rendered description changes. The stored variant and the
Codable encoding are untouched, so OCI content digests remain stable.

Testing

Added OCIPlatformTests cases for description consistency (equal arm64
platforms describe identically; arm64/v8 renders as linux/arm64;
arm/v7 and amd64 preserved). swift test --filter ContainerizationOCITests
passes (55 tests); swift format lint --strict clean.

Closes apple/container#1542 (normalization-consistency aspect).