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/front/tiny_unified_cache.d

@@ -17,7 +17,8 @@ core/front/tiny_unified_cache.o: core/front/tiny_unified_cache.c \
  core/tiny_region_id.h core/tiny_box_geometry.h core/ptr_track.h \
  core/hakmem_super_registry.h core/hakmem_tiny_superslab.h \
  core/superslab/superslab_inline.h core/hakmem_tiny.h core/hakmem_trace.h \
- core/hakmem_tiny_mini_mag.h core/front/../hakmem_tiny_superslab.h \
+ core/hakmem_tiny_mini_mag.h core/tiny_debug_api.h \
+ core/front/../hakmem_tiny_superslab.h \
  core/front/../superslab/superslab_inline.h \
  core/front/../box/pagefault_telemetry_box.h
 core/front/tiny_unified_cache.h:
@@ -51,6 +52,7 @@ core/superslab/superslab_inline.h:
 core/hakmem_tiny.h:
 core/hakmem_trace.h:
 core/hakmem_tiny_mini_mag.h:
+core/tiny_debug_api.h:
 core/front/../hakmem_tiny_superslab.h:
 core/front/../superslab/superslab_inline.h:
 core/front/../box/pagefault_telemetry_box.h: