cext: fix UB writing to uninitialized buffers in transpile_layout C API

[shofiqtest]

Summary

Fixes #15370.

As identified by @jakelishman in #15297 (comment), the three layout query functions in crates/cext/src/transpiler/transpile_layout.rs create a &mut [u32] slice over caller-provided uninitialized memory, which violates Rust's validity invariants even for write-only access.

Functions fixed:

• qk_transpile_layout_initial_layout
• qk_transpile_layout_output_permutation
• qk_transpile_layout_final_layout

Change

Replace std::slice::from_raw_parts_mut + iterator pattern with direct raw pointer writes using ptr.add(i).write(value):

// Before (UB — slice over uninitialized memory)
let out_slice = std::slice::from_raw_parts_mut(ptr, len);
out_slice.iter_mut().zip(data.iter()).for_each(|(dest, src)| *dest = src.0);

// After (safe — raw pointer write, no reference to uninitialized memory)
for (i, src) in data.iter().enumerate() {
    ptr.add(i).write(src.0);
}

The bug is only caught by Miri with -Zmiri-recursive-validation, which explains why existing tests pass.

Test

Built cleanly with cargo build -p qiskit-cext. Existing tests unchanged.

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[alexanderivrii]

Thanks for doing this, @shofiqtest. We already have an open PR for this (see #15375), but it looks like the contributor stopped working on it. If he does not reply within a few days, you are welcome to take over with this PR. Please see this comment #15375 (review), as it is relevant for this PR as well. In addition, please add LLM attribution to the PR summary.