Problem

CI fails on macos-latest (Debug tests + Release build) with:

error: no member named 'jthread' in namespace 'std'
  src/codec/lac/decoder.cpp:248
  src/codec/lac/encoder.cpp:392

std::jthread (C++20) is not exposed by the libc++ shipped on the current macos-latest image, while Linux (libstdc++) provides it — so Ubuntu and ASan/UBSan jobs pass and only macOS breaks. This is pre-existing on main (a docs-only PR surfaced it) and unrelated to that doc change.

Fix

• Switch the encoder and decoder worker pools from std::jthread to std::thread (the only two std::jthread uses; main.cpp already uses std::thread).
• Add an RAII WorkerJoinGuard that joins every started worker on scope exit, so a partial launch that throws during thread construction cannot leave joinable threads and call std::terminate. This preserves the exception-safe shutdown std::jthread gave for free and addresses the startup-safety concern in Make multithreaded encoder startup exception-safe #5.

Verification

Built and tested locally on macOS (the failing platform):

• cmake --build succeeds, no jthread references remain
• ctest: 2/2 suites pass
• lac_cli selftest: all sample-rate/bit-depth round-trips pass

Closes #5