fix(thread-pool): use std::call_once for thread-safe diagnostics init

[kcenon]

What

Summary

Replaces the unsynchronized lazy initialization of thread_pool::diagnostics_ with std::call_once to prevent a data race when multiple threads call diagnostics() concurrently for the first time.

Change Type

• Bugfix (fixes an issue)

Affected Components

• include/kcenon/thread/core/thread_pool.h — Add std::once_flag member
• src/impl/thread_pool/thread_pool.cpp — Both diagnostics() overloads

Why

Problem Solved

thread_pool::diagnostics() used a bare if (!diagnostics_) check followed by make_unique without any synchronization. Two threads calling diagnostics() simultaneously for the first time could both observe !diagnostics_ as true and both attempt allocation, causing a data race (undefined behavior). The const overload additionally used const_cast which is UB if the pool was originally declared const.

Related Issues

• Closes [Bug] thread_pool::diagnostics() lazy initialization is not thread-safe #668

Where

Files Changed

File	Type of Change
include/kcenon/thread/core/thread_pool.h	Add mutable std::once_flag diagnostics_init_flag_, #include <mutex>
src/impl/thread_pool/thread_pool.cpp	Replace bare if with std::call_once in both overloads
docs/API_REFERENCE.kr.md	Fix pre-existing broken TOC anchor

How

Implementation Details

// Before (TOCTOU race)
if (!diagnostics_) {
    diagnostics_ = std::make_unique<...>(*this);
}

// After (thread-safe)
std::call_once(diagnostics_init_flag_, [this]() {
    diagnostics_ = std::make_unique<...>(*this);
});

Breaking Changes

None — diagnostics() return type and behavior unchanged.

[github-actions]

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