Overlapping `memory.copy` (`copy_within`, dst > src) aborts the VM

Discovered in https://github.com/0xMiden/compiler/pull/1164

## Summary

wasm `memory.copy` (emitted by Rust `slice::copy_within`) has memmove semantics:
overlapping source/destination ranges are valid. The MASM lowering instead calls
a memcpy intrinsic that asserts the ranges do not overlap, so a valid wasm program
aborts at runtime (`assertion failed: source and destination ranges must not
overlap`) while the native build produces the correct memmove result.

## Reproduction

The `mem_overlap` test (currently `#[ignore]`d):

cargo test -p midenc-integration-tests mem_overlap -- --ignored

The issue is in the MASM lowering for the `memory.copy` instruction. Look at the code that handles this instruction, likely in a code generation or lowering module. The failing test `mem_overlap` in the integration tests provides a reproduction. Run the test to see the assertion failure, then find and replace the memcpy intrinsic with a proper memmove implementation. Verify by running the test after the fix.

Written by the indexing model from the issue text.