feat: add double-buffer context window reducer for chat history

Implements proactive context compaction using double buffering for the
ChatHistoryReducer system. Instead of stop-the-world summarization when
the context fills, this reducer begins checkpoint summarization at a
configurable threshold while the agent continues working, then swaps to
the pre-built back buffer seamlessly.

Three-phase algorithm:
1. Checkpoint: background summarization (asyncio.create_task) seeds back buffer
2. Concurrent: new messages go to both active and back buffers
3. Swap: back buffer becomes active context at swap threshold

Key features:
- Configurable checkpoint/swap thresholds (fraction of target_count)
- Non-blocking async checkpoint with timeout (default 120s)
- Stop-the-world fallback if no back buffer ready at swap time
- Incremental summary accumulation across generations
- Renewal policies: RECURSE (meta-summarize) or DUMP (clean restart)
- Graceful degradation on checkpoint failure

Reference: https://marklubin.me/posts/hopping-context-windows/

Author: marklubin

python/semantic_kernel/contents/__init__.py

@@ -7,6 +7,7 @@
 from semantic_kernel.contents.file_reference_content import FileReferenceContent
 from semantic_kernel.contents.function_call_content import FunctionCallContent
 from semantic_kernel.contents.function_result_content import FunctionResultContent
+from semantic_kernel.contents.history_reducer.chat_history_double_buffer_reducer import ChatHistoryDoubleBufferReducer
 from semantic_kernel.contents.history_reducer.chat_history_reducer import ChatHistoryReducer
 from semantic_kernel.contents.history_reducer.chat_history_summarization_reducer import ChatHistorySummarizationReducer
 from semantic_kernel.contents.history_reducer.chat_history_truncation_reducer import ChatHistoryTruncationReducer
@@ -35,6 +36,7 @@
     "AudioContent",
     "AuthorRole",
     "ChatHistory",
+    "ChatHistoryDoubleBufferReducer",
     "ChatHistoryReducer",
     "ChatHistorySummarizationReducer",
     "ChatHistoryTruncationReducer",