Modularize Warm Pool with 3 Box Refactorings - Phase B-3a Complete

Objective: Clean up warm pool implementation by extracting inline boxes
for statistics, carving, and prefill logic. Achieved full modularity
with zero performance regression using aggressive inline optimization.

Changes:

1. **Legacy Code Removal** (Phase 0)
   - Removed unused static __thread prefill_attempt_count variable
   - Cleaned up duplicate comments
   - Simplified carve failure handling

2. **Warm Pool Statistics Box** (Phase 1)
   - New file: core/box/warm_pool_stats_box.h
   - Inline APIs: warm_pool_record_hit/miss/prefilled()
   - All statistics recording externalized
   - Integrated into unified_cache.c
   - Performance: 0 cost (inlined to direct memory write)

3. **Slab Carving Box** (Phase 2)
   - New file: core/box/slab_carve_box.h
   - Inline API: slab_carve_from_ss()
   - Extracted unified_cache_carve_from_ss() function
   - Now reusable by other refill paths (P0, etc.)
   - Performance: 100% inlined, O(slabs) scan unchanged

4. **Warm Pool Prefill Box** (Phase 3)
   - New file: core/box/warm_pool_prefill_box.h
   - Inline API: warm_pool_do_prefill()
   - Extracted prefill loop with configurable budget
   - WARM_POOL_PREFILL_BUDGET = 3 (tunable)
   - Cold path optimization (only on empty pool)
   - Performance: Cold path cost (non-critical)

Architecture:
- core/front/tiny_unified_cache.c now 40+ lines shorter
- Logic distributed to 3 well-defined boxes
- Each box has single responsibility (SRP)
- Inline compilation preserves hot path performance
- LTO (-flto) enables cross-file inlining

Performance Results:
- 1M allocations: 4.099M ops/s (maintained)
- 5M allocations: 4.046M ops/s (maintained)
- 55.6% warm pool hit rate (unchanged)
- Zero regression on throughput
- All three boxes fully inlined by compiler

Code Quality Improvements:
✅ Removed legacy unused variables
✅ Separated concerns into specialized boxes
✅ Improved readability and maintainability
✅ Preserved performance via aggressive inline
✅ Enabled future reuse (carve box for P0)

Testing:
✅ Compilation: No errors
✅ Functionality: 1M and 5M allocation tests pass
✅ Performance: Baseline maintained
✅ Statistics: Output identical to pre-refactor

Next Phase: Consider similar modularization for:
- Registry scanning (registry_scan_box.h)
- TLS management (tls_management_box.h)
- Cache operations (unified_cache_policy_box.h)

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

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

core/front/tiny_unified_cache.d

@@ -32,6 +32,17 @@ core/front/tiny_unified_cache.o: core/front/tiny_unified_cache.c \
  core/front/../box/../superslab/superslab_types.h \
  core/front/../box/ss_slab_meta_box.h \
  core/front/../box/slab_freelist_atomic.h \
+ core/front/../box/warm_pool_stats_box.h \
+ core/front/../box/../hakmem_tiny_config.h \
+ core/front/../box/../front/tiny_warm_pool.h \
+ core/front/../box/slab_carve_box.h \
+ core/front/../box/../hakmem_tiny_superslab.h \
+ core/front/../box/../superslab/superslab_inline.h \
+ core/front/../box/../tiny_box_geometry.h \
+ core/front/../box/../box/pagefault_telemetry_box.h \
+ core/front/../box/warm_pool_prefill_box.h \
+ core/front/../box/../tiny_tls.h \
+ core/front/../box/../box/warm_pool_stats_box.h \
  core/front/../hakmem_env_cache.h
 core/front/tiny_unified_cache.h:
 core/front/../hakmem_build_flags.h:
@@ -85,4 +96,15 @@ core/front/../box/ss_tier_box.h:
 core/front/../box/../superslab/superslab_types.h:
 core/front/../box/ss_slab_meta_box.h:
 core/front/../box/slab_freelist_atomic.h:
+core/front/../box/warm_pool_stats_box.h:
+core/front/../box/../hakmem_tiny_config.h:
+core/front/../box/../front/tiny_warm_pool.h:
+core/front/../box/slab_carve_box.h:
+core/front/../box/../hakmem_tiny_superslab.h:
+core/front/../box/../superslab/superslab_inline.h:
+core/front/../box/../tiny_box_geometry.h:
+core/front/../box/../box/pagefault_telemetry_box.h:
+core/front/../box/warm_pool_prefill_box.h:
+core/front/../box/../tiny_tls.h:
+core/front/../box/../box/warm_pool_stats_box.h:
 core/front/../hakmem_env_cache.h: