False positive on AttributeDynamicLookup subscript

[rnapier]

Describe the bug
Periphery incorrectly reports subscript(dynamicMember:) as unused when extending AttributeDynamicLookup to support custom AttributeScope types.

When defining a custom AttributeScope and the corresponding AttributeDynamicLookup subscript (as documented by Apple for extending AttributedString), periphery reports:

warning: Unused function 'subscript(dynamicMember:)'

However, removing this subscript causes compilation to fail because self.myAttribute cannot be resolved on AttributeContainer.

Reproduction

import Foundation

// Custom attribute key
enum MyAttribute: AttributedStringKey {
    typealias Value = String
    static let name = "MyAttribute"
}

// Custom attribute scope - periphery reports this as unused
extension AttributeScopes {
    struct CustomAttributes: AttributeScope {
        let myAttribute: MyAttribute
    }
}

// Dynamic member lookup subscript - periphery reports this as unused
extension AttributeDynamicLookup {
    subscript<T: AttributedStringKey>(
        dynamicMember keyPath: KeyPath<AttributeScopes.CustomAttributes, T>
    ) -> T {
        self[T.self]
    }
}

// Extension that uses the custom attribute via dynamic member lookup
extension AttributeContainer {
    var customValue: String? {
        get { self.myAttribute }  // This requires the subscript and scope above
        set { self.myAttribute = newValue }
    }
}

// Main entry point
@main
struct Main {
    static func main() {
        var container = AttributeContainer()
        container.customValue = "hello"
        print(container.customValue ?? "nil")
    }
}

Output:

/private/tmp/periphery-false-positive/Sources/PeripheryFalsePositive/PeripheryFalsePositive.swift:18:5: warning: Unused function 'subscript(dynamicMember:)'

Environment
Copy the output from running:

3.4.0
swift-driver version: 1.127.14.1 Apple Swift version 6.2.3 (swiftlang-6.2.3.3.21 clang-1700.6.3.2)
Target: arm64-apple-macosx26.0
Xcode 26.2
Build version 17C52

[rnapier]

Investigating further, the issue is in DynamicMemberRetainer:

    for decl in graph.declarations(ofKind: .functionSubscript) {
        if decl.name == "subscript(dynamicMember:)", decl.parent?.attributes.contains(where: { $0.name == "dynamicMemberLookup" }) ?? false {
            graph.markRetained(decl)
        }

The filter on dynamicMemberLookup doesn't work for extensions. This would be a solution:

func mutate() throws {
    // Retain all subscript(dynamicMember:) declarations. This signature is specific to
    // @dynamicMemberLookup and may be declared in extensions of external types where we
    // cannot verify the attribute exists.
    for decl in graph.declarations(ofKind: .functionSubscript) {
        if decl.name == "subscript(dynamicMember:)" {
            graph.markRetained(decl)
        }
    }

If this seems right, I'll send it as a PR.