#8457 Preserve Tailscale access across Nightly Iroh upgrades

Summary

• preserve the historical legacy mobile listener default for dev and Nightly Macs when no preference has ever been written
• keep explicit current and legacy preferences authoritative, including an explicit off
• keep stable builds opt-in and retain the new iOS fail-closed rule for bearer tokens over raw Tailscale TCP

This is compatibility ingress only. An older iOS app can still reach an updated Nightly Mac, where the Mac performs same-account Stack authentication. A new iOS app still requires Iroh from a truly old Mac because that Mac has no cryptographic challenge that can attest a raw Tailscale route.

Verification

• regression-only commit 9a688df060 fails before the policy overload exists
• xcodebuild -project cmux.xcodeproj -scheme cmux-unit -configuration Debug -destination "platform=macOS" -derivedDataPath /tmp/cmux-tsmigr test -only-testing:cmuxTests/MobileHostServiceSettingsTests CODE_SIGNING_ALLOWED=NO (14 passed)
• swift test --package-path Packages/iOS/CmuxMobileTransport --filter CmxNetworkByteTransportFactorySecurityTests (5 passed)
• swift test --package-path Packages/iOS/CmuxMobileShell --filter IrohReconnectRouteSelectionTests (24 passed)
• cloud tagged build tsmigr succeeded: https://github.com/manaflow-ai/cmux/actions/runs/29676343291
• tagged runtime reported the legacy listener running and published both IPv4 and IPv6 Tailscale routes

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Summary by cubic

Preserves Tailscale pairing access across Nightly Iroh upgrades by keeping the legacy mobile listener on for dev/nightly builds when no preference exists. Explicit settings still win, and stable builds remain opt-in.

• Bug Fixes
  • Default listener depends on build flavor: dev/nightly on, stable off when no preference is set.
  • Explicit current or legacy preference overrides, including an explicit off.
  • Made build flavor explicit via isListeningEnabled(defaults:buildFlavor:) for deterministic behavior; added tests for nightly compatibility, explicit and legacy disables, and stable default.

Summary by CodeRabbit

• Bug Fixes
  • Fixed listener-enable behavior to consistently respect explicit current and legacy preferences.
  • Updated defaults by build type: nightly retains the prior behavior, while stable remains opt-in to avoid local network permission prompts.
• Tests
  • Added/expanded unit tests for build-flavor-specific defaults, including nightly behavior, explicit true/false handling, legacy overrides, and stable-build opt-in.