RFR: 8377044: Shenandoah: Convert ShenandoahHeap related code to use Atomic<T>

[duke]

On Wed, 11 Feb 2026 18:12:58 GMT, Ben Taylor <btaylor at openjdk.org> wrote:

> Modernizes the code as has been done in other areas of Shenandoah and G1 GCs.
> 
> The same tier1 tests pass before and after this change on x86_64 and aarch64 linux when run with `-XX:+UseShenandoahGC`

@benty-amzn 
Your change (at version 3c96217034f58ecaf5d0815b2f4314e584455bed) is now ready to be sponsored by a Committer.

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PR Comment: https://git.openjdk.org/jdk/pull/29676#issuecomment-3909769989