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_sample.py

@@ -2,7 +2,7 @@
 
 import torch
 
-from astrai.inference.sample import (
+from astrai.inference.runtime.sample import (
     FrequencyPenaltyStrategy,
     SamplingPipeline,
     TemperatureStrategy,
@@ -268,7 +268,7 @@ def test_sample_return_logprobs_matches_manual_computation():
     logits = torch.randn(2, 30)
     tokens, logprobs = sample(logits, temperature=0.7, top_p=0.95, return_logprobs=True)
     # Recompute with the same pipeline
-    from astrai.inference.sample import (
+    from astrai.inference.runtime.sample import (
         SamplingPipeline,
         TemperatureStrategy,
         TopPStrategy,