C SDK: Document thread safety requirements and fix js_cleanup

[clemensv]

Problem

The C SDK has thread safety concerns that need to be addressed:

Issues Found

1. Global allocator is unprotected - \g_allocator\ in \ ypes.c\ is mutable global state with no synchronization
2. \js_init()/\js_cleanup()\ are not thread-safe - Modifying global allocator during active validations causes undefined behavior
3. \js_cleanup()\ doesn't clear regex cache - Should call \js_regex_cache_clear()\

Current Behavior

• Validators are safe for concurrent use IF \js_init()\ is called once before any threads start
• \js_set_allocator()\ must NEVER be called while validators are active
• Each thread must use its own \js_result_t\ for results

Required Changes

1. Add thread safety documentation to README
2. Fix \js_cleanup()\ to clear the regex cache
3. Consider making \g_allocator\ immutable after initialization (optional)

Tests to Add

• Test concurrent validation from multiple threads (pthreads)
• Test that regex cache is cleared on \js_cleanup()\
• Test calling \js_init()\ multiple times is idempotent

Thread Safety Classification

Partially Thread-Safe - Safe with proper usage patterns documented

[clemensv]

Closing: superseded by issues that request actual thread-safety implementation, not just documentation.