Expose DurableServer callbacks to release upgrades

[jeregrine]

Problem

OTP's release handler discovers processes to suspend and pass through code_change/3 from each supervisor child specification's modules list. DurableServer children currently advertise only the generic DurableServer wrapper, so a release upgrade that changes a user callback module cannot associate that module with its running durable processes.

That leaves callback-owned runtime state outside the normal relup migration path.

Fix

Advertise both DurableServer and the user callback module in each durable child's modules list. The wrapper remains the process callback and continues delegating code_change/3 to the user module.

Update graceful shutdown child selection to recognize DurableServer by membership rather than requiring the old singleton module list.

Supporting information

This adds release-upgrade metadata only. It does not change discovery, restart eligibility, persistence, or shutdown status semantics.

Companion Sprites handoff change: https://github.com/superfly/sprites-api/pull/995