Implement Phase 2: Migrate to C++20

## Changes

### C++ Standard Update
- Updated CXX_STANDARD from 17 to 20 in CMakeLists.txt
- Applied to all targets:
  - PRTree extension module
  - benchmark_construction
  - benchmark_query
  - benchmark_parallel
  - stress_test_concurrent

### C++20 Compatibility Fixes
- Fixed lambda capture deprecation warning
- Changed `[=]` to `[this]` for explicit this capture
- Location: Leaf::filter() method

## Testing
- ✅ Compiles successfully with C++20
- ✅ No warnings or errors
- ✅ All targets build successfully

## Benefits of C++20
- Enables use of C++20 features in future phases:
  - std::span (Phase 8)
  - Concepts (Phase 8)
  - Ranges (Phase 8)
  - Three-way comparison (Phase 8)
- Better compiler optimizations
- Improved standard library

## Impact
- No API changes
- No performance impact expected
- Builds on modern compilers (GCC 10+, Clang 10+, MSVC 2019+)

Related: Phase 1 (thread safety)
Prepares for: Phase 8 (C++20 features)

Author: claude

CMakeLists.txt

@@ -70,7 +70,7 @@ target_link_libraries(PRTree PRIVATE
 )
 
 set_target_properties(PRTree PROPERTIES
-  CXX_STANDARD 17
+  CXX_STANDARD 20
   CXX_STANDARD_REQUIRED TRUE
   CXX_EXTENSIONS FALSE
   POSITION_INDEPENDENT_CODE ON
@@ -97,7 +97,7 @@ if(BUILD_BENCHMARKS)
     target_include_directories(benchmark_construction PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/cpp)
     target_link_libraries(benchmark_construction PRIVATE cereal snappy)
     set_target_properties(benchmark_construction PROPERTIES
-        CXX_STANDARD 17
+        CXX_STANDARD 20
         CXX_STANDARD_REQUIRED TRUE
         CXX_EXTENSIONS FALSE
     )
@@ -107,7 +107,7 @@ if(BUILD_BENCHMARKS)
     target_include_directories(benchmark_query PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/cpp)
     target_link_libraries(benchmark_query PRIVATE cereal snappy)
     set_target_properties(benchmark_query PROPERTIES
-        CXX_STANDARD 17
+        CXX_STANDARD 20
         CXX_STANDARD_REQUIRED TRUE
         CXX_EXTENSIONS FALSE
     )
@@ -117,7 +117,7 @@ if(BUILD_BENCHMARKS)
     target_include_directories(benchmark_parallel PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/cpp)
     target_link_libraries(benchmark_parallel PRIVATE cereal snappy)
     set_target_properties(benchmark_parallel PROPERTIES
-        CXX_STANDARD 17
+        CXX_STANDARD 20
         CXX_STANDARD_REQUIRED TRUE
         CXX_EXTENSIONS FALSE
     )
@@ -127,7 +127,7 @@ if(BUILD_BENCHMARKS)
     target_include_directories(stress_test_concurrent PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/cpp)
     target_link_libraries(stress_test_concurrent PRIVATE cereal snappy pthread)
     set_target_properties(stress_test_concurrent PROPERTIES
-        CXX_STANDARD 17
+        CXX_STANDARD 20
         CXX_STANDARD_REQUIRED TRUE
         CXX_EXTENSIONS FALSE
     )

cpp/prtree.h

@@ -285,7 +285,8 @@ template <class T, int B = 6, int D = 2> class Leaf {
   }
 
   bool filter(DataType<T, D> &value) { // false means given value is ignored
-    auto comp = [=](const auto &a, const auto &b) noexcept {
+    // Phase 2: C++20 requires explicit 'this' capture
+    auto comp = [this](const auto &a, const auto &b) noexcept {
       return a.second.val_for_comp(axis) < b.second.val_for_comp(axis);
     };