(feat) Add MemoryCoordinator and IndexRegistry for serverless scaling (#8)

Introduces two new components for serverless deployments:

MemoryCoordinator:
- Adaptive memory budget split between cache (40%) and mmap (60%)
- Automatic rebalancing based on pressure detection
- Workload hints (BulkIngestion, QueryHeavy, MemoryConstrained)
- Environment variable configuration (XTREE_MEMORY_BUDGET)

IndexRegistry:
- Lazy index loading - indexes registered as lightweight metadata
- On-demand loading via get_or_load<Record>()
- LRU-based cold index eviction under memory pressure
- Manifest integration for serverless cold start
- register_from_data_dir() for one-line initialization

Together these enable scaling to 1000+ indexes in memory-constrained
serverless environments (Lambda, Cloud Run) by loading only active
indexes and automatically unloading cold ones.

48 tests added, all passing.

Signed-off-by: Nicholas Walter Knize <nknize@apache.org>

Author: nknize

.gitignore

@@ -39,6 +39,7 @@ obj/
 .idea/
 *.iml
 .vscode/
+.claude/
 # But keep GitHub workflows
 !.github/
 *.swp

core/src/main/cpp/CMakeLists.txt

@@ -608,6 +608,8 @@ set(GTEST_SOURCES
     test/test_lru_sharded_stress.cpp  # Sharded LRU cache stress tests
     test/test_cache_policy.cpp  # Cache memory policy tests
     test/test_cache_policy_stress.cpp  # Cache policy stress tests
+    test/test_memory_coordinator.cpp  # Adaptive memory coordinator tests
+    test/test_index_registry.cpp  # Lazy index loading tests
     # Concurrent tests saved for adaptation to new persistence layer
     # test/test_xtree_concurrent_search.cpp
     # test/test_xtree_simple_concurrent.cpp

core/src/main/cpp/src/indexdetails.hpp

@@ -32,6 +32,7 @@
 #include "persistence/store_interface.h"
 #include "xtree_allocator_traits.hpp"  // For XAlloc
 #include "cache_policy.hpp"  // Cache memory policies
+#include "persistence/memory_coordinator.h"  // Adaptive memory coordination
 #include <memory>
 #include <mutex>
 #include <atomic>
@@ -111,9 +112,16 @@ namespace xtree {
                     break;
             }
 
-            std::cout << "[IndexDetails] Constructor completed for " 
+            std::cout << "[IndexDetails] Constructor completed for "
                       << (mode == PersistenceMode::DURABLE ? "DURABLE" : "IN_MEMORY") << " mode\n";
 
+            // Initialize memory coordinator for DURABLE mode
+            // This ensures adaptive memory budget is active when mmap is in use
+            if (mode == PersistenceMode::DURABLE) {
+                // Touch the singleton to ensure it's initialized with env vars
+                persist::MemoryCoordinator::global();
+            }
+
             // update cache size for each index
    //         for_each(IndexDetails<Record>::indexes.begin(),
    //                  IndexDetails<Record>::indexes.end(),
@@ -1070,6 +1078,9 @@ namespace xtree {
                       << " from dirty list of " << buckets.size() << " (iteration " << iteration << ")\n";
 #endif
             } // end while loop
+
+            // Tick the memory coordinator at safe points (after flush, no traversal in progress)
+            persist::MemoryCoordinator::global().tick();
         }
 
         /**