[Concurrency] Define Swift 6 UI-rendering boundaries

[NachoSoto]

Scope

Part 2 of 4 in the Swift 6 migration. This PR contains production correctness changes only:

• make render data and image-inflater content mode Sendable
• isolate UIKit view rendering and view-construction callbacks to the main actor
• route deferred UI work through UIScheduler

Dependencies

• Base: #84 — package configuration
• Follow-up: the next PR adds the narrowly scoped escape hatches required for non-Sendable ReactiveSwift callback state.

This branch intentionally depends on its follow-up for a complete Swift 6 build; review it for the actor-boundary design.

Validation

• swiftlint --config .swiftlint.yml --strict
• git diff --check