Fix SharedVectorHeader alignment mismatch on ARM32 platforms #10098
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Add explicit 8-byte alignment to SharedVectorHeader on both Rust and C++ sides to fix layout corruption on ARM Cortex-M7 (32-bit). Without explicit alignment, the struct has 4-byte natural alignment on ARM32, but pointer arithmetic calculations differ between Rust and C++, causing a 4-byte offset when accessing SharedVector data. This corrupts all layout calculations (HorizontalLayout, VerticalLayout, GridLayout) and makes Slint unusable on ARM32 embedded platforms. The fix adds alignas(8) to C++ and #[repr(C, align(8))] to Rust, ensuring consistent struct layout across the FFI boundary on all platforms. Fixes: slint-ui#10097
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Add explicit 8-byte alignment to SharedVectorHeader on both Rust and C++ sides to fix layout corruption on ARM Cortex-M7 (32-bit).
Without explicit alignment, the struct has 4-byte natural alignment on ARM32, but pointer arithmetic calculations differ between Rust and C++, causing a 4-byte offset when accessing SharedVector data. This corrupts all layout calculations (HorizontalLayout, VerticalLayout, GridLayout) and makes Slint unusable on ARM32 embedded platforms.
The fix adds alignas(8) to C++ and #[repr(C, align(8))] to Rust, ensuring consistent struct layout across the FFI boundary on all platforms.
Fixes: #10097