watchOS unit-lane observation path for hostability helpers

[schatt]

Summary

PlatformContainerStructureAssertions.isHostable (and UIKit Form/Section/VStack fallbacks) are gated #if canImport(UIKit) && !os(watchOS), so on watchOS they return false. That makes hostability smokes unusable on the watch unit lane.

Goal

Add a cheap, truthful watchOS observation path (or an explicit skip/unavailable contract) so watch unit tests do not claim hostability they cannot observe.

Options (not prescriptive)

• Host via WatchKit / SwiftUI hosting APIs available on watchOS Simulator
• Type-name / mirror-only checks where hosting is unavailable
• Mark hostability helpers unavailable on watch and route tests to pure-logic asserts instead

Out of scope

First-class watchOS product support → #377.

Parent

Follow-up from #219 (foundational secondary-platform test honesty). Related: #233.

[schatt]

Parent #219 closed after landing foundational helpers + toolbar tip into next (eeef6aac). This issue is the carve-out for remaining work.