[AlexanderSoftCode]

Context

Ruff reported 49 findings, none on recently touched code, and several docstrings
rendered badly in editor hovers. This PR clears the lint backlog and fixes the
docstrings. It also adds a package docstring, because help(aether) and hover
showed nothing useful with the lazy-loaded public names.

Three commits:

• chore(lint): mostly mechanical (ruff --fix for unused imports, typing syntax,
  nested with, import order). Review the test files with git diff -w.
• docs: docstring-only.
• docs(aether): docstring-only.

Lint

Auto-fixed: unused imports, Optional/Union/Tuple annotations, nested with
statements, a redundant return None, import order.

Fixed by hand:

• adam_base_suite.py: removed a try/except Exception that only called
  self.fail. A kernel failure now surfaces as a test error with its traceback.
• model_base_suite.py: target_device was computed but the same expression was
  repeated inline. Both model.to(...) calls now use the variable.
• test_max_pool2d.py: dropped an unused layer =.
• batchnorm_kernel.py: variant: str | None = None.

Ignored on purpose:

• RUF022 for *.pyi: stub __all__ lists are grouped by topic, not sorted.
• F401 and PIE790 for the integration test skeleton: its placeholder imports
  and pass bodies are intentional.
• TRY004 on Loss.validate_graph: inline noqa, because its ValueError is a
  tested public error type.

Docstrings

The language server's hover joins nested bullets under an Args entry into one
paragraph, and shows ** as literal asterisks. The verbose entries in
Model.train() and Model.evaluate() now describe their modes inline, and the
**Label:** markers in Notes sections are plain labels (9 places across 4 files).

aether/__init__.py and aether/__init__.pyi get a short package docstring: what
the framework is, lazy loading, and that the backend is chosen with Model.to().
It leaves out class lists and the version, which the lazy maps, stub and package
metadata already cover.

Behavior changes

None.

Testing

• python3 -m unittest discover tests: 1158 tests pass (72 skipped), run in the
  ROCm devcontainer.
• ruff check . passes.
• No new tests: the changes are lint and docstrings, with no behavior change.
• The hover rendering in Zed was not re-checked after the docstring edits.

Follow-ups (out of scope)

• Two tests fail on Windows; fixing them is the next PR.
• pyproject.toml still says 1.1.0. The version bump for the next release goes
  in the Windows-fix PR.
• Benchmarks.