`CoerceShared` unsoundness due to not checking `repr`.

The following code causes a segmentation fault at run time. The compiler seems to be incorrectly assuming that MyMut and MyRef have the same size.

#![feature(reborrow)]

use std::marker::{Reborrow, CoerceShared};

#[repr(align(4096))]
struct MyMut<'a>(&'a i32);

impl Reborrow for MyMut<'_> {}

#[derive(Clone, Copy)]
#[repr(align(65536))]
struct MyRef<'a>(&'a i32);

impl<'a> CoerceShared<MyRef<'a>> for MyMut<'a> {}

fn main() {
    foo(MyMut(&1));
}

fn foo(x: MyMut<'_>) {
    bar(x);
}

fn bar(x: MyRef<'_>) {}

cc @aapoalas

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Reproducible on the playground with version 1.97.0-nightly (2026-05-10 4b0c9d76ae7d387229ca)

Start by reproducing the provided minimized example with the stated nightly compiler and trace how the compiler handles the CoerceShared implementation and the two differently aligned types. Identify the missing repr compatibility check, then add a regression test showing that the unsound coercion no longer compiles or runs unsafely.