Add module READMEs, .gitignore, and remove Bin/ from tracking

Add README.md for each library module documenting API, platform
backends, and usage. Add .gitignore for build artifacts. Remove
pre-built binaries from version control.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

libsst-concurrency/README.md

@@ -0,0 +1,30 @@
+# libsst-concurrency
+
+Cross-platform threading and synchronization primitives in C, with C++ RAII
+wrappers in ZUtil.
+
+## Primitives
+
+| Primitive | C API prefix | C++ wrapper |
+|---|---|---|
+| Threads | `SST_Concurrency_*Thread*` | `ZThread` |
+| Mutexes | `SST_Concurrency_*Mutex*` | `ZMutex` / `ZLock` |
+| Events | `SST_Concurrency_*Event*` | `ZEvent` |
+| Semaphores | `SST_Concurrency_*Semaphore*` | `ZSemaphore` |
+| Thread-local storage | `SST_Concurrency_*TLS*` | -- |
+| Read-write locks | `SST_Concurrency_*ReadWrite*` | `ZReadWriteLock` |
+| One-time init | `SST_Concurrency_ExecOnce` | -- |
+| Thread barriers | `SST_Concurrency_*ThreadBarrier*` | -- |
+
+## Platform backends
+
+| Primitive | POSIX | Win32 | Solaris | macOS |
+|---|---|---|---|---|
+| Thread | pthread | `_beginthreadex` | `thr_*` | pthread |
+| Mutex | `pthread_mutex_t` | `CRITICAL_SECTION` | `mutex_t` | pthread |
+| Event | `pthread_cond_t` | `CreateEvent` | `cond_t` | pthread |
+| Semaphore | `sem_t` | `CreateSemaphore` | `sema_t` | Custom (cond+mutex) |
+| TLS | `pthread_key_t` | `TlsAlloc` | `thr_keycreate` | pthread |
+
+Read-write locks and thread barriers use atomic spinlocks on all platforms
+(depends on libsst-atomic).