fix: prevent subscription model encode and notification races

[nan-li]

Summary

• Lands Domus's #1685 on current main (cherry-picked cleanly; no conflict with SDK-4874 executor changes).
• Makes OSSubscriptionModel mutations and archival race-safe: lock-protected value-type State, atomic snapshot() for encode, mutationLock so each setter updates state and fires delta/observer notifications in order.
• Closes production crashes SDK-4899 / #1588; may also address #1587 (verify after ship).

Test plan

• UserConcurrencyTests on iPhone 17 Pro (Full test plan, -parallel-testing-enabled NO) — 10/10 passed, including:
  • testEncodingSubscriptionModel_withConcurrentMutation_usesConsistentSnapshot
  • testConcurrentSubscriptionChangesNotifyInMutationOrder
• Confirm Reduced plan still skips UserConcurrencyTests (pre-existing); consider running Full in CI for this PR or a follow-up.
• After merge, ask Domus to verify SDK-4899 on a patch build.

Credit: @chunhuidu-dev / Domus for the original fix in #1685.

Made with Cursor

[nan-li]

Supersedes fork PR #1685 (same commit, rebased onto current main).