[Concurrency] Constrain Swift 6 escape hatches

[NachoSoto]

Scope

Part 3 of 4 in the Swift 6 migration. This PR contains only the explicit production escape hatches needed around legacy ReactiveSwift state:

• mark the disposable accessed from UIKit deinitialization as nonisolated(unsafe)
• wrap the non-Sendable ReactiveSwift observer and lifetime in a tightly scoped @unchecked Sendable box before the main-actor callback

Dependencies

• Base: #85 — production concurrency boundaries
• Follow-up: test-only Swift 6 migrations and cleanup will base on this PR.

The unsafe annotations are intentionally isolated here so reviewers can audit their scope independently from the actor-boundary changes.

Validation

• iOS Simulator library build succeeds
• swiftlint --config .swiftlint.yml --strict
• git diff --check