Closes #360
The solid native navigation-bar option is already implemented on main in 7de71de. This follow-up makes the issue acceptance criteria executable and documents the public option:
- covers opaque fill with a large title and after switching back to a non-large title
- verifies all four UIKit appearance slots remain populated and opt-out restores defaults
- documents the option and its push/pop/tab-swipe behavior in the Native STX guide
- keeps the generated-source assertion for the opt-out path
Validation: bun test packages/ios/src/index.test.ts (107 pass, 0 fail). Simulator/Xcode mobile validation remains CI-owned.