Implement thread-safe libc with proper locking

[pbalduino]

Goal

Add thread-safety to global libc state with proper locking to enable safe concurrent use of C standard library.

Context

Currently, libc has no concurrency protection:

• stdio buffers (FILE* streams) aren't locked
• Environment variable access isn't protected
• Allocator (malloc/free) may not be thread-safe
• Global state (errno, strtok static buffer, etc.) has no protection

With pthread support (#109), programs will crash or corrupt data when multiple threads use libc simultaneously.

Definition of Done

• [ ] Thread-safe stdio (flockfile/funlockfile, per-FILE mutexes)
• [ ] Thread-safe environment (getenv/putenv/setenv locking)
• [ ] Thread-safe allocator (malloc/free locking if needed)
• [ ] Thread-local errno
• [ ] Reentrant variants (_r functions: strtok_r, localtime_r, etc.)
• [ ] Testing with multithreaded programs

Dependencies

Required

• #109 - pthread API and POSIX threading

Related

• #110 - Thread-safe C library (parent issue, may be duplicate)

Priority

Medium - Required after pthread implementation

Estimated Time

6-8 weeks part-time