Add SwiftUI hosting smoke tests for field-level Observation

[thliu21]

Summary

• add Apple-platform SwiftUI hosting smoke tests for two independent projected fields
• verify a field B emission does not reevaluate the field A-only subtree on Observation-capable systems
• verify global invalidation and the ObservableObject fallback both invalidate the expected hosted subtrees
• make view identity, shared observation setup, and teardown cancellation explicit
• document supported Xcode versions and simulator destinations

Fixes #18

Harness

Each projected field is rendered by a separate SwiftUI View identity. The
runtime path uses KMPObservedObject, while a test-only configuration forces
the same production store through its ObservableObject fallback.

The source emits dependency notifications synchronously. Tests wait for
positive body-render and cancellation signals; timeouts are failure bounds
only. The field-isolation assertion is made after the B subtree has rendered,
without an inverted expectation or sleep.

The suite also includes a runtime-gated iOS 15/16 fallback test. A forced
fallback test keeps that contract covered on current runtimes when an older
simulator is unavailable.

Validation

• SwiftUI hosting suite: 3 passed, 1 expected runtime skip, 0 failures
• repeated hosting run: 3 passed, 1 expected runtime skip, 0 failures
• complete strict-concurrency compilation
• API compatibility: no breaking changes in all three products
• package manifest consistency
• whitespace validation

The repository-wide -warnings-as-errors command currently stops on three
pre-existing Swift 6.3 WeakMutability diagnostics in
KMPObservableBridgeTests.swift; the new hosting test file compiles without
diagnostics.

Verified locally with Xcode 26.5 on macOS 26.5.2. An iOS 15/16 runtime was not
installed locally, so that runtime-specific case remains explicitly
unverified rather than being reported as passed.