fix(ptr): replace deprecated Boost atomic count

[IvanaGyro]

Problem

intrusive_ptr_base.hpp includes Boost's deprecated boost/detail/atomic_count.hpp, and tons of deprecation warning occupy the CI logs.

Fix

Replace the internal reference counter with std::atomic<int> while preserving the existing intrusive-pointer reference-count operations.

Testing

Not run locally at request; CI will verify.

Closes #1121.

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.92%. Comparing base (a969a66) to head (0799293).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1166   +/-   ##
=======================================
  Coverage   67.92%   67.92%           
=======================================
  Files         208      208           
  Lines       22355    22355           
  Branches       72       72           
=======================================
  Hits        15185    15185           
  Misses       7148     7148           
  Partials       22       22           

Flag	Coverage Δ
cpp	67.99% <ø> (ø)
python	63.96% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
C++ backend	67.99% <ø> (ø)
Python bindings	∅ <ø> (∅)
Python package	63.96% <ø> (ø)

Files with missing lines	Coverage Δ
include/intrusive_ptr_base.hpp	16.66% <ø> (ø)

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