## 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
```swift
// == 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.
We had a couple spots we were using this logic, probably better to shove
it in a function. The other thing this fixes is today .current was only
checking for arm64 which on a linux host uname with return aarch64, so
we'll fatalError trying to use it today..
Vitor Hugo