ENH: Refactor the ThrottledMessenger class to use an internal thread as a timer

[imikejackson]

Threaded Throttled Messenger - Change Summary

Problem

The existing ThrottledMessenger::sendThrottledMessage() called
std::chrono::steady_clock::now() on every invocation in tight voxel loops,
adding 50-200ns of pure overhead per call. For a 10M voxel loop, this meant
0.5-2.0 seconds of wasted time on clock syscalls alone. Additionally, the spdlog
async backend introduced unnecessary complexity with redundant throttle layers.

Solution

A singleton background timer thread (MessageDispatcher) now handles all clock
checking and string formatting. The filter's hot path (sendMessage()) only
stores raw argument values -- no clock check, no fmt::format() call.

Performance Improvement

Operation	Old Cost	New Cost
Hot-path call (single usize)	~50-200ns (clock syscall)	~10ns (2 atomic stores, lock-free)
Hot-path call (multi-arg)	~50-200ns (clock syscall)	~15-25ns (mutex + atomic store)
String construction	Every call (if throttle passes)	Only on timer tick (~every 1s)

Architecture

Filter Thread (hot loop)              Background Timer Thread (100ms tick)
  |                                     |
  |  sendMessage(args...)               |
  |    -> storeArgs()  [~10-25ns]       |
  |    -> set atomic flag               |
  |                                     |  timerLoop() wakes
  |                                     |    -> iterate channels
  |                                     |    -> check atomic flag (fast bail)
  |                                     |    -> check interval elapsed
  |                                     |    -> fmt::format(args...)
  |                                     |    -> call MessageHandler

Key Classes

Class	Role
MessageHelper	Top-level factory; creates messengers from IFilter::MessageHandler
ThrottledMessenger<Args...>	User-facing handle; sendMessage() stores args cheaply
ThrottledChannel<Args...>	Internal channel; mutex+tuple storage for general case
ThrottledChannel<usize>	Atomic specialization; lock-free for the most common case
ProgressHelper	Multi-threaded progress tracking factory
ProgressWorker	Per-thread handle with atomic incrementProgress()
ProgressChannel	Internal channel; std::atomic<usize> progress counter
MessageDispatcher	Singleton; background timer thread flushes channels every 100ms
IThrottledChannel	Type-erased interface for channel registration
callable_traits<T>	Template metaprogramming for formatter argument deduction

Files Changed

Core Implementation (2 files, rewritten)

• src/simplnx/Utilities/MessageHelper.hpp -- All public API classes
• src/simplnx/Utilities/MessageHelper.cpp -- MessageDispatcher singleton + pimpl

New Files (2 files)

• test/MessageHelperTest.cpp -- 10 Catch2 unit tests
• docs/SendingFilterUpdates.md -- Developer documentation

Build System (3 files)

• CMakeLists.txt -- Removed find_package(spdlog) and link dependency
• vcpkg.json -- Removed spdlog from dependencies
• test/CMakeLists.txt -- Added MessageHelperTest.cpp

Migrated Callers (37 files)

Core Utilities (7 files)

• src/simplnx/Utilities/SegmentFeatures.hpp
• src/simplnx/Utilities/SegmentFeatures.cpp
• src/simplnx/Utilities/SampleSurfaceMesh.hpp
• src/simplnx/Utilities/SampleSurfaceMesh.cpp
• src/simplnx/Utilities/OStreamUtilities.cpp
• src/simplnx/Utilities/AlignSections.hpp
• src/simplnx/Utilities/AlignSections.cpp

SimplnxCore Filter Algorithms (17 files)

• Filters/Algorithms/FillBadData.cpp
• Filters/Algorithms/ErodeDilateBadData.cpp
• Filters/Algorithms/RequireMinimumSizeFeatures.cpp
• Filters/Algorithms/ComputeNeighborhoods.hpp
• Filters/Algorithms/ComputeNeighborhoods.cpp
• Filters/Algorithms/ComputeArrayStatistics.cpp
• Filters/Algorithms/ComputeVertexToTriangleDistances.cpp
• Filters/Algorithms/CalculateTriangleGroupCurvatures.hpp
• Filters/Algorithms/CalculateTriangleGroupCurvatures.cpp
• Filters/Algorithms/RemoveFlaggedFeatures.cpp
• Filters/Algorithms/DBSCAN.cpp
• Filters/Algorithms/ReadStlFile.cpp
• Filters/Algorithms/FeatureFaceCurvature.cpp
• Filters/Algorithms/ComputeNeighborListStatistics.cpp
• Filters/Algorithms/ComputeArrayHistogramByFeature.cpp
• Filters/Algorithms/AlignSectionsFeatureCentroid.cpp
• Filters/Algorithms/WriteLAMMPSFile.cpp

SimplnxCore Filter Files (2 files)

• Filters/IterativeClosestPointFilter.cpp
• Filters/ComputeFeatureNeighborsFilter.cpp

OrientationAnalysis Plugin (6 files)

• Filters/Algorithms/ComputeGBCD.cpp
• Filters/Algorithms/RotateEulerRefFrame.cpp
• Filters/Algorithms/NeighborOrientationCorrelation.cpp
• Filters/Algorithms/BadDataNeighborOrientationCheck.cpp
• Filters/Algorithms/ComputeKernelAvgMisorientations.cpp
• Filters/Algorithms/AlignSectionsMisorientation.cpp

SimplnxReview Plugin (3 files, separate repository)

• Filters/Algorithms/GroupMicroTextureRegions.cpp
• Filters/Algorithms/ComputeGroupingDensity.cpp
• Filters/Algorithms/MergeColonies.cpp

What Was Removed

• spdlog dependency (library, includes, vcpkg entry, CMake linkage)
• Messenger class (hpp + cpp) and Messenger::Impl
• MessageHandlerSink (spdlog custom sink)
• ThrottleSink (spdlog rate-limiting sink)
• ThreadPool singleton (spdlog async thread pool)
• ThrottledMessageFunctor concept
• ProgressMessageFunctor concept
• ProgressMessageData struct
• Old non-templated ThrottledMessenger class
• ProgressMessenger class
• ProgressMessageHelper class

What Was Retained

• IFilter::Message, IFilter::ProgressMessage, IFilter::MessageHandler -- unchanged
• CalculatePercentComplete<T>() -- retained as-is

Migration Pattern

// OLD:
MessageHelper messageHelper(m_MessageHandler, throttleRate);
auto messenger = messageHelper.createThrottledMessenger(throttleRate);
messenger.sendThrottledMessage([&]() {
  return fmt::format("{}/{}", current, total);
});

// NEW:
MessageHelper messageHelper(m_MessageHandler);
auto messenger = messageHelper.createThrottledMessenger(
    [total](usize current) {
      return fmt::format("{}/{}", current, total);
    });
messenger.sendMessage(current);

Testing

• 10 new MessageHelper unit tests: all pass
• 58 tests for migrated filter files: 100% pass
• Full test suite: 936/1071 pass (87%), all 135 failures are pre-existing
• Filter cancellation: verified compatible, zero coupling to shouldCancel