refactor: 移除 SPLADE 和 hybrid_cascade，精简搜索架构

删除 SPLADE 稀疏神经搜索后端和 hybrid_cascade 策略，
将搜索架构从 6 种后端简化为 4 种（FTS Exact/Fuzzy, Binary Vector, Dense Vector, LSP）。

主要变更：
- 删除 splade_encoder.py, splade_index.py, migration_009 等 4 个文件
- 移除 config.py 中 SPLADE 相关配置（enable_splade, splade_model 等）
- DEFAULT_WEIGHTS 改为 FTS 权重 {exact:0.25, fuzzy:0.1, vector:0.5, lsp:0.15}
- 删除 hybrid_cascade_search()，所有 cascade fallback 改为 self.search()
- API fusion_strategy='hybrid' 向后兼容映射到 binary_rerank
- 删除 CLI index_splade/splade_status 命令和 --method splade
- 更新测试、基准测试和文档

codex-lens/tests/api/test_semantic_search.py

@@ -427,12 +427,12 @@ class TestFusionStrategyMapping:
 
         mock_engine.binary_cascade_search.assert_called_once()
 
-    def test_hybrid_strategy_calls_hybrid_cascade_search(self):
-        """Test that hybrid strategy maps to hybrid_cascade_search."""
+    def test_hybrid_strategy_maps_to_binary_rerank(self):
+        """Test that hybrid strategy maps to binary_rerank_cascade_search (backward compat)."""
         from codexlens.api.semantic import _execute_search
 
         mock_engine = MagicMock()
-        mock_engine.hybrid_cascade_search.return_value = MagicMock(results=[])
+        mock_engine.binary_rerank_cascade_search.return_value = MagicMock(results=[])
         mock_options = MagicMock()
 
         _execute_search(
@@ -444,7 +444,7 @@ class TestFusionStrategyMapping:
             limit=20,
         )
 
-        mock_engine.hybrid_cascade_search.assert_called_once()
+        mock_engine.binary_rerank_cascade_search.assert_called_once()
 
     def test_unknown_strategy_defaults_to_rrf(self):
         """Test that unknown strategy defaults to standard search (rrf)."""