[ntoskrnl7]

Summary

• implement thread_pool::queue_item::cancel() for tasks that have not started running
• add queue_cancellable() so callers can obtain a queue_item<T> and cancel pending work
• complete canceled task futures with ext::thread_pool::task_canceled
• make worker initializer failure stop startup and return false from start()
• keep finalizers tied to workers that successfully initialized
• document the cancellable queue and initializer failure contract

Verification

• git diff --check
• Markdown fence/link checks for README.md and docs/**/*.md
• direct compile/run smoke test with c++ -std=c++17 -pthread -Iinclude
• /tmp/cmake-3.30.5/bin/cmake -S test -B /tmp/ntoskrnl7-ext-build-thread-cancel -DCMAKE_BUILD_TYPE=Release
• /tmp/cmake-3.30.5/bin/cmake --build /tmp/ntoskrnl7-ext-build-thread-cancel --config Release -j 2
• /tmp/cmake-3.30.5/bin/ctest --test-dir /tmp/ntoskrnl7-ext-build-thread-cancel -C Release --output-on-failure