docs(cli): document dp test stress output and dp show serialization-tree

Parent issue: #5917 (checklist items 4–5)

## Scope

1. **`dp test` stress output** (`330fa7558`, `feat: dp test stress`): when the tested data contains stress labels and the model predicts virial/stress, `dp test` writes per-frame stress details to a `.s.out` file and prints Stress MAE/RMSE in eV/ų. Nothing about stress appears in `doc/test/test.md`.
2. **`dp show serialization-tree`** (`e19bd188b`): `deepmd/main.py:1049` adds a `serialization-tree` choice, implemented in `deepmd/entrypoints/show.py:153` via `DeepEval.serialize()`. `doc/model/show-model-info.md` does not list it.

## Implementation plan

### 1. `doc/test/test.md` — new "Evaluate stress" section

Read `deepmd/entrypoints/test.py` first, in particular:

- lines ~500–560: where the stress detail file `.s.out` is written and the requirement "Stress detail output requires both reference and prediction stresses";
- lines ~815–840 and ~895–910, ~970–980: Stress MAE/RMSE printing.

Then document:

- When stress labels are present in the test data, `dp test` additionally reports `Stress MAE` and `Stress RMSE` in eV/ų in the summary output (and averaged over systems).
- The per-frame stress detail is written to `.s.out` (analogous to `.e.out`/`.f.out`/`.v.out`); list its columns as implemented (verify ordering in code).
- Convention: stress sigma = −virial / volume, tensile-positive, in eV/ų (verify against `test.py:825`).

### 2. `doc/model/show-model-info.md` — add `serialization-tree`

Read `deepmd/entrypoints/show.py:140-170` and the `DeepEval.serialize` implementation to determine which backends/model kinds support it.

- Add `serialization-tree` to the `` bullet list (after `observed-type`): shows the model's serialization tree (the structure of the data serialized into the model file).
- Add a matching **serialization-tree** entry under "Output Description".
- Note backend support explicitly only if it is restricted (verify; `DeepEval.serialize` suggests all backends exposing `DeepEval`).

## Constraints

- Verify file naming (`.s.out`) and column layout directly from `deepmd/entrypoints/test.py`; do not guess.
- Match the existing documentation style of both pages.

## Verification

- `grep -n -i "stress" doc/test/test.md` shows the new section.
- `grep -n "serialization-tree" doc/model/show-model-info.md` shows the attribute listed.

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Parent issue: #5917

Coding agent: opencode
opencode version: 1.18.9
Model: ustc/k3
Reasoning effort: max