bug(pt-expt): compiled DPA2 lower miscompiles descriptors that depend on atype_ext

## Summary

Making a DPA2/DPA3 or shared descriptor path depend on `atype_ext` (e.g. by masking `em`/`diff`/`sw` with `atype_ext >= 0` inside `EnvMat.call`) produces **wrong forces** when the model is run through the **compiled pt_expt DPA2 lower**.

## Reproduction context

This was hit while developing deepmd/dpmodel/utils/env_mat.py for #5833 (commit 6d67a1257). The naive fix of re-masking the zeroed virtual-center outputs using `atype_ext` made the descriptor read `atype_ext`, and the pt_expt DPA2 compiled (`torch.compile` / graph-lowered `call_lower`) path returned incorrect forces; forces were correct in the eager/dense path. This is why #5833 chose to neutralize the `davg`/`dstd` normalization instead of re-masking, keeping `EnvMat.call` independent of `atype_ext`.

## Impact / risk

- A future developer who adds an `atype_ext` dependency to a descriptor for a reasonable reason could silently reintroduce wrong forces when a compiled DPA2 model happens to use that descriptor.
- Nothing in CI currently pins this behavior; there is no regression test or assertion.

## Suggested follow-ups

- Characterize the pt_expt DPA2 lower miscompilation (which transformation of the `atype_ext`-dependent branch breaks) and add a regression test on the compiled lower.
- At minimum, add a comment/assertion documenting that `EnvMat.call` must remain `atype_ext`-independent.

Related: #5833