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/tiny_region_id.h

@@ -20,6 +20,7 @@
 #include "hakmem_super_registry.h"
 #include "superslab/superslab_inline.h"
 #include "hakmem_tiny.h"  // For TinyTLSSLL type
+#include "tiny_debug_api.h"  // Guard/failfast declarations
 
 // Feature flag: Enable header-based class_idx lookup
 #ifndef HAKMEM_TINY_HEADER_CLASSIDX
@@ -266,8 +267,6 @@ static inline void* tiny_region_id_write_header(void* base, int class_idx) {
     // ========== END ALLOCATION LOGGING ==========
 
     // Optional guard: log stride/base/user for targeted class
-    extern int tiny_guard_is_enabled(void);
-    extern void tiny_guard_on_alloc(int cls, void* base, void* user, size_t stride);
     if (tiny_guard_is_enabled()) {
         size_t stride = tiny_stride_for_class(class_idx);
         tiny_guard_on_alloc(class_idx, base, user, stride);
@@ -311,7 +310,6 @@ static inline int tiny_region_id_read_header(void* ptr) {
         }
 #endif
         // Optional guard hook for invalid header
-        extern void tiny_guard_on_invalid(void* user_ptr, uint8_t hdr);
         if (tiny_guard_is_enabled()) tiny_guard_on_invalid(ptr, header);
         return -1;
     }