fix(Platform): make arm64 nil-variant and v8 hash identically

[Otoru]

Summary

Platform.== treats arm64 with nil variant as equal to arm64/v8, but hash(into:) used description which serializes them differently (linux/arm64 vs linux/arm64/v8). This violates the Hashable contract — equal values must produce the same hash.

Root cause

// == returns true for these two
let a = Platform(arch: "arm64", os: "linux", variant: nil)
let b = Platform(arch: "arm64", os: "linux", variant: "v8")
a == b // true ✓

// but hash was different — broken
a.hashValue == b.hashValue // false ✗ (before this fix)

This mismatch caused Set<Platform> and Dictionary<Platform, ...> lookups to silently miss entries when one platform was decoded from JSON (no variant field in the manifest) and another was created via Platform(from:) or Platform.current (which both set variant = "v8").

Practical consequence

In apple/container, this manifests as inconsistent platform-string normalization across stages of a single container build — some stages log linux/arm64, others linux/arm64/v8 — which can cause COPY --from=<stage> to fail to resolve the source stage under concurrent builds. See apple/container#1542.

Fix

hash(into:) now normalizes arm64 with nil variant to "v8" before hashing, matching the existing == behavior.