Fix macOS CI checksums and Helgrind lifecycle

[etr]

Summary

• use the portable shasum -a 256 -c form for pinned libmicrohttpd downloads
• replace the blocking-start pthread mutex/cond pair with C++20 atomic wait/notify
• remove daemon_lifecycle pthread ownership and update its state documentation

Validation

• local macOS build and targeted ws_start_stop / daemon_lifecycle suites pass
• minimal Ubuntu 24.04 Valgrind reproduction reproduces the exact three CI Helgrind signatures before the lifecycle change
• pinned libmicrohttpd checksum verifies with shasum -a 256 -c on macOS
• GitHub Actions validation follows in this PR

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.63%. Comparing base (02bf41c) to head (41ec471).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #378      +/-   ##
==========================================
- Coverage   70.09%   69.63%   -0.47%     
==========================================
  Files          80       80              
  Lines        4237     4228       -9     
  Branches     1521     1520       -1     
==========================================
- Hits         2970     2944      -26     
- Misses        366      372       +6     
- Partials      901      912      +11     

Files with missing lines	Coverage Δ
src/detail/daemon_lifecycle.cpp	54.05% <100.00%> (-1.99%)	⬇️
src/httpserver/detail/daemon_lifecycle.hpp	100.00% <ø> (ø)
src/httpserver/detail/webserver_impl.hpp	100.00% <ø> (ø)
src/webserver.cpp	59.68% <100.00%> (-0.20%)	⬇️

... and 9 files with indirect coverage changes

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