Third Phase 1D cluster. `UIScreen.mainScreen.nativeScale` chain reads
through `#objc_call(*void)` + `#objc_call(f64)`. First standalone
`#objc_call(f64)` exercise — `f64` returns had only been covered
indirectly by the 4×f64 UIEdgeInsets HFA path. Net -4 lines.
iOS-sim chess regression smoke: sharp text rendering + accurate touch
hit-testing both confirm `plat.dpi_scale` is being populated correctly
through the new path.
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