This implements safer vsnprintf/snprintf with C99 semantics and rigorous bounds checking to eliminate stack buffer overflow vulnerabilities.

• Add vsnprintf() with proper size parameter and bounds checking
• Add snprintf() wrapper for formatted string output
• Remove unsafe sprintf() and vsprintf() functions completely
• Refactor printf() to use vsnprintf with fixed 256-byte buffer
• Migrate all application code to snprintf (pipes_small, mqueues, pipes_struct)
• Add comprehensive libc test suite with 31 tests covering:
  • Basic formatting (%s, %d, %u, %x, %p, %c, %%)
  • Edge cases (size=0, size=1, truncation)
  • C99 semantics (return values, null termination)
  • Buffer boundary protection
  • ISR safety (reentrancy, no malloc, bounded execution)

Summary by cubic

Replaced unsafe formatting with bounded snprintf/vsnprintf (C99) to fix stack buffer overflows and guarantee null-termination. Refactored printf and app code; added a libc test suite for edge cases and truncation.

• Bug Fixes

  • Eliminates stack buffer overruns via bounds-aware writes.
  • Enforces C99 return values and null-termination when size > 0.
  • Reentrant and ISR-safe; no dynamic allocation.

• Refactors

  • Added vsnprintf/snprintf; removed sprintf/vsprintf.
  • printf now uses vsnprintf with a fixed 256-byte buffer.
  • Migrated apps to snprintf (pipes_small, mqueues, pipes_struct); added test_libc and Makefile entry.

^{Written for commit 2137698. Summary will update automatically on new commits.}