visionOS unit-lane evidence for #219 secondary-platform helpers

[schatt]

Summary

Most #219 green evidence ran on tvOS (plus some iOS ViewInspector). visionOS unit-lane verification of the same helpers/contracts is still thin.

Goal

Re-run and extend representative #219 coverage on visionOS Simulator unit lane:

• PlatformContainerStructureAssertions (Form/Section/VStack + isHostable)
• PlatformTypographyTestAssertions wired suites
• PlatformToolbarPlacementTests platform contracts
• Drop-in / capability smokes already converted

Post xcresult evidence on this issue (scheme, destination, pass counts).

Out of scope

First-class visionOS product support → #378.

Parent

Follow-up from #219. Related: #233.

[schatt]

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