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_debug_api.h

@@ -0,0 +1,60 @@
+// tiny_debug_api.h - Debug/Guard/Failfast API declarations
+// Purpose: Central header for debug instrumentation functions
+//          Eliminates implicit declaration warnings across codebase
+// License: MIT
+// Date: 2025-11-29
+
+#ifndef TINY_DEBUG_API_H
+#define TINY_DEBUG_API_H
+
+#include <stdint.h>
+#include <stddef.h>
+
+// Forward declarations for types used in function signatures
+struct SuperSlab;
+struct TinySlabMeta;
+
+// ============================================================================
+// Guard System API (defined in hakmem_tiny_ace_guard_box.inc)
+// ============================================================================
+
+// Check if guard instrumentation is enabled
+// Controlled by HAKMEM_TINY_GUARD environment variable
+int tiny_guard_is_enabled(void);
+
+// Called on allocation to track block metadata
+// cls: size class, base: slab base, user: user pointer, stride: block size
+void tiny_guard_on_alloc(int cls, void* base, void* user, size_t stride);
+
+// Called when invalid header is detected
+// user_ptr: pointer with bad header, hdr: actual header value found
+void tiny_guard_on_invalid(void* user_ptr, uint8_t hdr);
+
+// ============================================================================
+// Fail-Fast Debugging API (defined in tiny_failfast.c)
+// ============================================================================
+
+// Get current fail-fast level (0=disabled, 1=log, 2=signal, 3=abort)
+// Controlled by HAKMEM_TINY_REFILL_FAILFAST environment variable
+int tiny_refill_failfast_level(void);
+
+// Log fail-fast event with slab metadata
+// stage: operation name, class_idx: size class, ss: SuperSlab pointer
+// meta: slab metadata, ptr: block pointer, prev: previous freelist pointer
+void tiny_failfast_log(const char* stage,
+                       int class_idx,
+                       struct SuperSlab* ss,
+                       struct TinySlabMeta* meta,
+                       void* ptr,
+                       void* prev);
+
+// Abort on fail-fast condition (or log/signal based on level)
+// stage: operation name, ss: SuperSlab pointer, slab_idx: slab index
+// ptr: problematic pointer, reason: error description
+void tiny_failfast_abort_ptr(const char* stage,
+                             struct SuperSlab* ss,
+                             int slab_idx,
+                             void* ptr,
+                             const char* reason);
+
+#endif // TINY_DEBUG_API_H