[taplin]

This repo just transferred from taplin/Perfect-AuthNet. This PR gives it the same baseline as the other org packages and fixes the Linux build.

• The package never built on Linux. AuthNetClient.swift and the tests used URLSession/HTTPURLResponse without importing FoundationNetworking. They now import it under #if canImport(FoundationNetworking), as Perfect-FileMaker-DataAPI does.
• Apache 2.0 LICENSE, as in the other PerfectlySoft repos. The repo had no license; the old README said to treat it as all rights reserved.
• CI: Linux Swift 6.2 and 6.4, macOS Xcode 26.0.1 and 26.6, debug and release, warnings as errors (the Perfect-Logger workflow).
• Deployment target lowered from macOS 26 to macOS 12. The only newer-looking API is async URLSession.data(for:), which is available from macOS 12.
• README rewritten for the org: install from the PerfectlySoft URL (from: "1.0.0"), Linux listed as supported, and the internal project-status text and storefront-specific example removed.

Tests: 14 offline on macOS and Linux. The live sandbox tests are still gated behind AUTHNET_TESTS=1 and weren't run.

Verified from a clean .build: macOS debug tests and release builds (Xcode, warnings as errors), and Linux tests on Swift 6.2 and 6.4 (swift:*-noble containers, warnings as errors). The README samples compile against the package in a scratch consumer. This PR is the first GitHub CI run.

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