os-usque 0.2_19

Updates the bundled usque-nativetun port to upstream 0.8.3 at main merge commit a5d87348d86c66cd90d8cc0b838d8dac6871e824. Existing registrations with a known different client version now perform one existing-key registration refresh at process startup, persist the version only after Cloudflare accepts it, and retry on a later start without blocking tunnel operation if the refresh fails. Existing egress client, Mesh node, browser-assisted, legacy, and Access service-token enrollment behavior is preserved. Validation: 38 plugin unit tests, PHP syntax checks, full FreeBSD optimized release build, stage-qa, check-plist, package builds, and package metadata checks.