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

`Platform.description` rendered the same arm64 platform two different ways
depending on how the value was constructed: `linux/arm64` when the variant
was `nil`, and `linux/arm64/v8` when the variant was set to `"v8"`. These
are the same platform — `==`, `hash`, and Set membership already treat an
arm64 `nil` variant as equivalent to `"v8"` — so two equal values produced
different descriptions and drifted between `arm64` and `arm64/v8` across
stages of a single build (apple/container#1542).

Omit the redundant `v8` variant for arm64 so equal platforms always
describe as `linux/arm64`, matching how Docker and containerd display it.
Only the rendered description changes; the stored variant and Codable
encoding are untouched, so OCI content digests remain stable.

Author: Halvanhelv

Sources/ContainerizationOCI/Platform.swift

@@ -47,15 +47,27 @@ public struct Platform: Sendable, Equatable {
         return .init(arch: normalized.arch, os: "linux", variant: normalized.variant)
     }
 
-    /// The computed description, for example, `linux/arm64/v8`.
+    /// The computed description, for example, `linux/amd64` or `linux/arm/v7`.
+    ///
+    /// `arm64`'s only defined variant is `v8`, which `==` and `hash` already treat as
+    /// equivalent to a `nil` variant. The redundant `v8` is therefore omitted so that
+    /// two equal arm64 platforms (one with `variant == nil`, one with `"v8"`) describe
+    /// identically as `linux/arm64`, rather than drifting between `arm64` and
+    /// `arm64/v8`. See apple/container#1542.
     public var description: String {
         let architecture = architecture
-        if let variant = variant {
+        if let variant, !Self.isRedundantVariant(variant, for: architecture) {
             return "\(os)/\(architecture)/\(variant)"
         }
         return "\(os)/\(architecture)"
     }
 
+    /// Whether `variant` is the canonical default for `architecture` and can be omitted
+    /// from the rendered description without losing information.
+    private static func isRedundantVariant(_ variant: String, for architecture: String) -> Bool {
+        architecture == "arm64" && variant == "v8"
+    }
+
     /// The CPU architecture, for example, `amd64` or `arm64`.
     public var architecture: String {
         Self.normalizeArch(_rawArch).arch

Tests/ContainerizationOCITests/OCIPlatformTests.swift

@@ -81,4 +81,40 @@ struct OCIPlatformTests {
         set.insert(withoutVariant)
         #expect(set.contains(withV8), "arm64/v8 must be found in a Set that contains arm64 with nil variant")
     }
+
+    // MARK: - description consistency (issue apple/container#1542)
+
+    @Test func arm64_nilAndV8_sameDescription() {
+        let withoutVariant = Platform(arch: "arm64", os: "linux", variant: nil)
+        let withV8 = Platform(arch: "arm64", os: "linux", variant: "v8")
+        #expect(
+            withoutVariant.description == withV8.description,
+            "equal arm64 platforms must produce the same description"
+        )
+    }
+
+    @Test func arm64_descriptionDropsRedundantV8() {
+        let withV8 = Platform(arch: "arm64", os: "linux", variant: "v8")
+        #expect(withV8.description == "linux/arm64", "arm64/v8 is canonical arm64, rendered without the redundant variant")
+    }
+
+    @Test func arm64_nilVariantDescription() {
+        let withoutVariant = Platform(arch: "arm64", os: "linux", variant: nil)
+        #expect(withoutVariant.description == "linux/arm64")
+    }
+
+    @Test func arm64_fromStringWithV8DescriptionIsCanonical() throws {
+        let parsed = try Platform(from: "linux/arm64/v8")
+        #expect(parsed.description == "linux/arm64", "parsing arm64/v8 then describing must yield the canonical short form")
+    }
+
+    @Test func arm_v7_descriptionKeepsVariant() {
+        let armv7 = Platform(arch: "arm", os: "linux", variant: "v7")
+        #expect(armv7.description == "linux/arm/v7", "non-redundant variants such as arm/v7 must be preserved")
+    }
+
+    @Test func amd64_descriptionUnaffected() {
+        let amd64 = Platform(arch: "amd64", os: "linux")
+        #expect(amd64.description == "linux/amd64")
+    }
 }