refactor(cmake): remove legacy directory fallback from monitoring_system library (#681)

The library definition at the root carried two parallel branches:

1. Outer if(EXISTS .../kcenon/monitoring AND EXISTS .../src) gating a
   'new directory structure' build path.
2. Inner if(SOURCE_COUNT GREATER 0) gating the GLOB result, with an
   else() that hardcoded 11 cpp files (Fall back to legacy structure).
3. Outer else() that hardcoded the same 11 cpp files (Use legacy
   directory structure).

After PR #680 consolidated src/impl into the canonical feature
directories, the legacy branches can never trigger:

- include/kcenon/monitoring and src/ both always exist.
- The GLOB always finds sources (33 files at the time of writing).
- The hardcoded fallback list is a stale subset of the real source
  set; activating it would silently miss most of the library.

Replace the dual-structure scaffolding with a single GLOB-based
add_library() call. CMakeLists.txt drops 57 lines (996 -> 939) and
the build now has exactly one path, mirroring the EPIC #674 goal of
collapsing the legacy fallback before further CMake decomposition.

No behavioural change for current builds (the GLOB path was already
the live one). The hardcoded source lists in the deleted fallback
branches no longer reflect the actual src/ layout, so removing them
also eliminates a drift hazard.

Author: kcenon

CMakeLists.txt

@@ -514,88 +514,31 @@ if(MONITORING_ENABLE_UBSAN)
     target_link_options(monitoring_system_interface INTERFACE -fsanitize=undefined)
 endif()
 
-# Check for new structure
+# Source layout (canonical, post src/impl consolidation)
 set(MONITORING_INCLUDE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/include)
 set(MONITORING_SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/src)
 
-if(EXISTS ${MONITORING_INCLUDE_DIR}/kcenon/monitoring AND EXISTS ${MONITORING_SOURCE_DIR})
-    message(STATUS "Monitoring System: Using new directory structure")
-
-    # Collect source files from new structure
-    file(GLOB_RECURSE MONITORING_HEADERS
-        ${MONITORING_INCLUDE_DIR}/kcenon/monitoring/*.h
-    )
-
-    file(GLOB_RECURSE MONITORING_SOURCES
-        ${MONITORING_SOURCE_DIR}/*.cpp
-    )
-
-    # Check if new structure has sources
-    list(LENGTH MONITORING_SOURCES SOURCE_COUNT)
-    if(SOURCE_COUNT GREATER 0)
-        # Create library with new structure
-        add_library(monitoring_system STATIC
-            ${MONITORING_SOURCES}
-            ${MONITORING_HEADERS}
-        )
+# Collect source files
+file(GLOB_RECURSE MONITORING_HEADERS
+    ${MONITORING_INCLUDE_DIR}/kcenon/monitoring/*.h
+)
 
-        target_include_directories(monitoring_system
-            PUBLIC
-                $<BUILD_INTERFACE:${MONITORING_INCLUDE_DIR}>
-                $<INSTALL_INTERFACE:include>
-            PRIVATE
-                $<BUILD_INTERFACE:${MONITORING_SOURCE_DIR}>
-        )
-    else()
-        # Fall back to legacy structure
-        message(STATUS "Monitoring System: New structure incomplete, using src sources")
-        add_library(monitoring_system STATIC
-            src/context/thread_context.cpp
-            src/core/distributed_tracer.cpp
-            src/core/performance_monitor.cpp
-            src/core/thread_local_buffer.cpp
-            src/core/central_collector.cpp
-            src/core/adaptive_monitor.cpp
-            src/collectors/container_collector.cpp
-            src/platform/linux_metrics.cpp
-            src/platform/windows_metrics.cpp
-            src/platform/cgroup_metrics.cpp
-            src/platform/docker_metrics.cpp
-        )
+file(GLOB_RECURSE MONITORING_SOURCES
+    ${MONITORING_SOURCE_DIR}/*.cpp
+)
 
-        target_include_directories(monitoring_system
-            PUBLIC
-                $<BUILD_INTERFACE:${MONITORING_INCLUDE_DIR}>
-                $<INSTALL_INTERFACE:include>
-            PRIVATE
-                $<BUILD_INTERFACE:${MONITORING_SOURCE_DIR}>
-        )
-    endif()
-else()
-    # Use legacy structure
-    message(STATUS "Monitoring System: Using legacy directory structure")
-    add_library(monitoring_system STATIC
-        src/context/thread_context.cpp
-        src/core/distributed_tracer.cpp
-        src/core/performance_monitor.cpp
-        src/core/thread_local_buffer.cpp
-        src/core/central_collector.cpp
-        src/core/adaptive_monitor.cpp
-        src/collectors/container_collector.cpp
-        src/platform/linux_metrics.cpp
-        src/platform/windows_metrics.cpp
-        src/platform/cgroup_metrics.cpp
-        src/platform/docker_metrics.cpp
-    )
+add_library(monitoring_system STATIC
+    ${MONITORING_SOURCES}
+    ${MONITORING_HEADERS}
+)
 
-    target_include_directories(monitoring_system
-        PUBLIC
-            $<BUILD_INTERFACE:${MONITORING_INCLUDE_DIR}>
-            $<INSTALL_INTERFACE:include>
-        PRIVATE
-            $<BUILD_INTERFACE:${MONITORING_SOURCE_DIR}>
-    )
-endif()
+target_include_directories(monitoring_system
+    PUBLIC
+        $<BUILD_INTERFACE:${MONITORING_INCLUDE_DIR}>
+        $<INSTALL_INTERFACE:include>
+    PRIVATE
+        $<BUILD_INTERFACE:${MONITORING_SOURCE_DIR}>
+)
 
 target_link_libraries(monitoring_system PUBLIC
     monitoring_system_interface