Cleanup: Add tiny_debug_api.h to eliminate guard/failfast implicit warnings

Created central header for debug instrumentation API to fix implicit
function declaration warnings across the codebase.

Changes:
1. Created core/tiny_debug_api.h
   - Declares guard system API (3 functions)
   - Declares failfast debugging API (3 functions)
   - Uses forward declarations for SuperSlab/TinySlabMeta

2. Updated 3 files to include tiny_debug_api.h:
   - core/tiny_region_id.h (removed inline externs)
   - core/hakmem_tiny_tls_ops.h
   - core/tiny_superslab_alloc.inc.h

Warnings eliminated (6 of 11 total):
✅ tiny_guard_is_enabled()
✅ tiny_guard_on_alloc()
✅ tiny_guard_on_invalid()
✅ tiny_failfast_log()
✅ tiny_failfast_abort_ptr()
✅ tiny_refill_failfast_level()

Remaining warnings (deferred to P1):
- ss_active_add (2 occurrences)
- expand_superslab_head
- hkm_ace_set_tls_capacity
- smallmid_backend_free

Impact:
- Cleaner build output
- Better type safety for debug functions
- No behavior changes

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

core/hakmem_tiny_tls_ops.h

@@ -6,6 +6,7 @@
 #include "hakmem_super_registry.h"
 #include "tiny_remote.h"
 #include "box/tiny_next_ptr_box.h"
+#include "tiny_debug_api.h"  // Guard/failfast declarations
 #include <stdint.h>
 
 // Forward declarations for external dependencies