Replace Cython with nanobind for Python bindings

[pablogsal]

The Cython-based bindings required extensive marshaling between Python
and C++ data structures, causing performance overhead and maintenance
complexity. Each call crossed the language boundary multiple times,
converting maps, thread lists, and frame data back and forth.

This migration moves to nanobind with scikit-build-core for the build
system. The key architectural change is moving logic that previously
lived in Cython or Python into C++: maps parsing, version detection,
and thread construction now happen entirely in C++ before returning
results to Python. This eliminates round-trips and simplifies the
codebase by removing the Cython layer entirely. The Python API remains
unchanged.

Issue number of the reported bug or feature request: #

Describe your changes
A clear and concise description of the changes you have made.

Testing performed
Describe the testing you have performed to ensure that the bug has been addressed, or that the new feature works as planned.

Additional context
Add any other context about your contribution here.

[lkollar]

Just a couple of comments. I did not go through all of the changes (it's a lot 😆).

[codecov-commenter]

Codecov Report

❌ Patch coverage is 78.01932% with 182 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.33%. Comparing base (fbc18c1) to head (a32e415).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pystack/_pystack/bindings.cpp	82.42%	61 Missing ⚠️
src/pystack/_pystack/version_detector.cpp	41.93%	54 Missing ⚠️
src/pystack/_pystack/maps_parser.cpp	82.46%	27 Missing ⚠️
src/pystack/_pystack/logging.cpp	55.31%	21 Missing ⚠️
src/pystack/_pystack/thread_builder.cpp	90.54%	7 Missing ⚠️
src/pystack/_pystack/unwinder.cpp	56.25%	7 Missing ⚠️
src/pystack/_pystack/pyframe.cpp	20.00%	4 Missing ⚠️
src/pystack/_pystack/pythread.cpp	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #272      +/-   ##
==========================================
- Coverage   83.07%   79.33%   -3.74%     
==========================================
  Files          46       51       +5     
  Lines        6211     5508     -703     
  Branches      470      568      +98     
==========================================
- Hits         5160     4370     -790     
- Misses       1051     1138      +87     

Flag	Coverage Δ
cpp	79.33% <78.01%> (-3.74%)	⬇️
python	79.33% <78.01%> (?)
python_and_cython	?

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.