Fix Sparkle update packaging and prepare v0.2.6

[yvette-carlisle]

Summary

• Strip local Xcode/CommandLineTools Swift toolchain rpaths from staged Rsnap.app before signing.
• Add a staging gate so release packaging fails if local Swift toolchain rpaths leak into RsnapNativeHost.
• Treat Sparkle downloads as off whenever automatic checks are disabled, and cover the mode matrix in the native-host probe.
• Bump workspace version to 0.2.6 for the release tag.

Validation

• cargo make fmt-toml
• cargo make checks
• cargo make test-host-reset
• cargo make test-macos-native-host-stage
• scripts/smoke/sparkle-update-local.sh --self-check
• staged Rsnap.app reports CFBundleShortVersionString=0.2.6 and CFBundleVersion=0.2.6
• staged RsnapNativeHost rpaths read back as /usr/lib/swift, @loader_path, and @executable_path/../Frameworks