Add stack overflow detection mechanism #14

Problem

Currently, stackman provides stack switching but no built-in overflow detection. Users must manually implement guard pages or canaries.

Proposal

Add optional stack overflow detection helpers:

1. Guard zone utilities:

   • stackman_guard_init(void *stack, size_t size, size_t guard_bytes) - Initialize guard pattern
   • stackman_guard_check(void *stack, size_t size, size_t guard_bytes) - Verify guard intact

2. Stack usage tracking:

   • stackman_stack_usage(void *stack_base, size_t size) - Measure actual usage via stack painting

Benefits

• Easier debugging of stack-related crashes
• Production overflow detection
• Helps users size stacks appropriately

Implementation

Could be a separate header (stackman_debug.h) to keep core library minimal.

Related

test_04 already uses basic guard bytes for validation - could formalize this pattern.