test: deduplicate suites and prune low-value cases

- extract shared helpers for dataset writers, scheduler construction, thread interleaving, hf roundtrips, and moe configs
- remove about 20 cases whose only assertions were format checks, restated declarations, fake-taxonomy duplicates, or test-local scaffolding
- strengthen weak cases into exact reference comparisons, positional mask checks, and deterministic outcomes
- replace two schedule factory smoke tests with cosine/sgdr formula assertions
- delete root-level CLI tests whose merge-priority facts are covered by tests/config/test_cli.py
- suite shrinks from 857 to 826 items; ruff format, import order, and pytest all green

tests/extension/test_linear_dispatch.py

@@ -6,7 +6,6 @@ import torch
 import torch.nn.functional as F
 
 from astrai.extension import is_available, linear
-from astrai.extension.backend import linear as public_linear
 from astrai.extension.dispatch import explain, op_backend, resolve
 
 # The package attribute ``linear`` is the dispatched function; reach the
@@ -36,10 +35,6 @@ def _routes_to_gemv(monkeypatch, x, weight, bias=None) -> bool:
     return linear(x, weight, bias) is sentinel
 
 
-def test_linear_backend_is_public():
-    assert linear is public_linear
-
-
 def test_model_linear_routes_through_backend(monkeypatch):
     sentinel = torch.randn(2, 4)