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

astrai/inference/cache/__init__.py

@@ -0,0 +1,27 @@
+"""KV cache subsystem: buffers, strategies, pool management."""
+
+from astrai.inference.cache.buffer import KVCache, KVStorage, ReqToTokenPool
+from astrai.inference.cache.pool import PagePool, TaskCacheManager, page_hash
+from astrai.inference.cache.strategy import (
+    AllocationStrategy,
+    Allocator,
+    ContiguousStrategy,
+    PagedStrategy,
+    RadixCache,
+    TaskCacheState,
+)
+
+__all__ = [
+    "KVCache",
+    "KVStorage",
+    "ReqToTokenPool",
+    "Allocator",
+    "RadixCache",
+    "TaskCacheState",
+    "AllocationStrategy",
+    "ContiguousStrategy",
+    "PagedStrategy",
+    "PagePool",
+    "TaskCacheManager",
+    "page_hash",
+]