refactor: split infer core into subpackages by concern

- Eliminate core/ directory into cache/, runtime/, network/ subpackages plus flat modules
- Split cache.py (647 lines) into cache/{buffer,strategy,pool}.py by layer
- Add explicit ContiguousStrategy, make AllocationStrategy a real ABC
- Move TaskCacheState to cache/strategy.py, drop string forward references
- Rename api/ to network/, server.py to app.py
- Move sample.py into runtime/ alongside executor and graph
- Simplify TaskCacheManager.__init__ to single pool param
- Expose pool.strategy and pool.req_pool as public properties
- Fix KVCache import in attention_backend.py (TYPE_CHECKING guard)
- Fix steady-state decode reading uninitialized position_ids on first step

tests/inference/test_scheduler.py

@@ -38,8 +38,8 @@ def test_scheduler_concurrent_add_task(mock_model_and_tokenizer):
     """Test concurrent add_task operations."""
     mock_model, mock_tokenizer = mock_model_and_tokenizer
 
-    with patch("astrai.inference.core.scheduler.AutoModel"):
-        with patch("astrai.inference.core.scheduler.AutoTokenizer"):
+    with patch("astrai.inference.scheduler.AutoModel"):
+        with patch("astrai.inference.scheduler.AutoTokenizer"):
             scheduler = InferenceScheduler(
                 model=mock_model,
                 tokenizer=mock_tokenizer,
@@ -77,8 +77,8 @@ def test_scheduler_concurrent_add_remove_task(mock_model_and_tokenizer):
     """Test concurrent add and remove task operations."""
     mock_model, mock_tokenizer = mock_model_and_tokenizer
 
-    with patch("astrai.inference.core.scheduler.AutoModel"):
-        with patch("astrai.inference.core.scheduler.AutoTokenizer"):
+    with patch("astrai.inference.scheduler.AutoModel"):
+        with patch("astrai.inference.scheduler.AutoTokenizer"):
             scheduler = InferenceScheduler(
                 model=mock_model,
                 tokenizer=mock_tokenizer,
@@ -126,8 +126,8 @@ def test_scheduler_concurrent_get_stats(mock_model_and_tokenizer):
     """Test concurrent get_stats operations."""
     mock_model, mock_tokenizer = mock_model_and_tokenizer
 
-    with patch("astrai.inference.core.scheduler.AutoModel"):
-        with patch("astrai.inference.core.scheduler.AutoTokenizer"):
+    with patch("astrai.inference.scheduler.AutoModel"):
+        with patch("astrai.inference.scheduler.AutoTokenizer"):
             scheduler = InferenceScheduler(
                 model=mock_model,
                 tokenizer=mock_tokenizer,