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-os/README.md

@@ -0,0 +1,49 @@
+# libsst-os
+
+Cross-platform operating system abstractions in C.
+
+## Subsystems
+
+### Memory (`SST_Alloc`, `SST_SysMem`)
+- Aligned allocation/free, safe malloc/realloc with overflow checking
+- Page-level allocation, protection (`R/W/X`), and size queries
+
+### File I/O (`SST_File`)
+- Open, read, write, seek, flush, close
+- Async and sequential/random hints
+
+### File system (`SST_FileSys`)
+- Directory listing, create, remove
+
+### Memory-mapped files (`SST_Mmap`)
+- Create/destroy mappings, sync to disk
+
+### Dynamic libraries (`SST_DynLib`)
+- Load, symbol lookup, close
+- OS-specific name generation (`.dll` / `.so` / `.dylib`)
+
+### CPU info (`SST_CPU`, `SST_CPUCache`)
+- Physical/logical CPU counts, affinity get/set
+- Cache line size, data cache flush, instruction cache invalidate
+- Cache ops in per-architecture assembly (x86, ARM, IA-64, MIPS, PPC, SPARC)
+
+### Timing (`SST_Time`)
+- Microsecond, millisecond, and floating-point second resolution
+
+### Endian (`SST_Endian`)
+- Byte-swap 16/32/64, host-to-BE/LE conversion macros
+
+### User info (`SST_User`)
+- Username, real name, home directory
+
+### Safe arithmetic (`SST_SafeArithmetic`)
+- Overflow-checked add/multiply for all integer widths
+
+### Assertions (`SST_Assert`)
+- Debug and runtime assertion macros with pluggable handlers
+
+## Platform backends
+
+Each subsystem has separate `_Win32.c` and `_POSIX.c` implementations, with
+additional Solaris and macOS variants where needed. CPU cache operations are
+in per-architecture assembly files.