[pull] master from deepmodeling:master - #321

## Summary

- preserve the source artifact's concrete lower-input semantics during
backend conversion instead of resolving `lower_kind="auto"`
- expose `lower_input_kind` from `.pte` and `.pt2` metadata so exported
artifacts retain their lower across subsequent conversions
- reject graph-to-dense-only conversions rather than silently changing
the model function
- document that graph semantics require a graph-native training and
freeze workflow

## Root cause

`dp convert-backend` always passed `lower_kind="auto"` to the pt_expt
serializer. A dense-trained DPA1 model was therefore reinterpreted as
graph-native whenever the reconstructed target model advertised graph
support. Dense padding contributes `-davg/dstd` when `davg` is nonzero,
while the graph lower contains no padding edges, so the generated
artifact represented a different function.

## Verification

- targeted conversion and metadata tests: 9 passed
- PTE serialization round-trip: 1 passed
- real nonzero-`davg` `.pth` to `.pt2` conversion selected
`lower_input_kind=nlist`
- source versus converted artifact: energy delta 0, force max delta
8.882e-16, virial max delta 6.661e-16
- Ruff, diff checks, and all pre-commit hooks passed

Closes #5973

Related to #5862 and #5824.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Backend conversions preserve supported lower-input representations,
including dense, graph, neighbor-list, and canonical formats.
* Model exports across supported frameworks now include lower-input
representation metadata.
  * Legacy files without metadata use compatible automatic defaults.
* Unsupported target representations are rejected to prevent silent
semantic changes.
* Schema-neutral model containers retain lower-input metadata for later
conversion.

* **Documentation**
* Added guidance on conversion behavior, preservation rules, and
supported representations.

* **Tests**
* Expanded coverage for metadata preservation, legacy artifacts,
serialization formats, and conversion validation.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>