fix(connectivity): stabilize Lockdown heartbeat and DDI readiness

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Summary

Stabilize the on-device connectivity lifecycle for Lockdown pairing while preserving the established Remote Pairing/RSD flow. This prevents transient DDI and profile-management failures caused by child-service work racing heartbeat readiness.

Root cause

• Lockdown DDI and profile operations could begin before the first successful Marco/Polo exchange.
• The device heartbeat interval was used directly as the receive timeout, leaving no scheduling margin and producing false disconnects.
• Deferred startup DDI work could be lost when a tunnel endpoint replacement created a new connectivity generation.
• Heartbeat clients were recreated instead of being owned persistently by the active Lockdown session.
• Remote Pairing could be incorrectly coupled to Lockdown heartbeat readiness.
• DDI detection did not consistently recognize legacy and personalized mount records.

Changes

• Add protocol-aware, generation-scoped connectivity session state.
• Use one persistent heartbeat client per Lockdown generation.
• Mark the session Ready only after the first complete Marco/Polo exchange.
• Gate Lockdown startup DDI work on Heartbeat Ready.
• Use the device interval plus a five-second heartbeat receive margin.
• Preserve deferred startup DDI intent across endpoint generation changes.
• Revoke Lockdown child-service readiness after heartbeat loss and revalidate sessions after endpoint or foreground changes.
• Keep Remote Pairing on its existing RSD path without a heartbeat gate.
• Recognize legacy /Developer + DiskImageType=Developer and personalized /System/Developer DDI records.
• Fail Lockdown DDI and profile writes fast while heartbeat is not ready; writes with indeterminate outcomes are not retried blindly.

Pairing behavior

Pairing mode	Heartbeat	Startup DDI
Lockdown	One persistent session heartbeat; Ready after Marco/Polo	Deferred until the current session is Ready
Remote Pairing	No Lockdown heartbeat	Existing RSD flow without a heartbeat gate

Validation

Manually passed on physical iPhone 13 devices:

• iOS 16.0.2 using Lockdown
• iOS 17.5.1 using Remote Pairing/RSD
• Health Check remained stable during a 60-second observation
• DDI recognition and application refresh/install completed successfully
• No Heartbeat(Timeout), UnexpectedEof, ImageMountFailed, or transient dmg=false was observed during the final Lockdown validation

Automated validation: swift test passed 14 tests with 0 failures.

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