ENV cleanup: Remove obsolete ULTRAHOT variable + organize docs

Changes:
1. Removed HAKMEM_TINY_FRONT_ENABLE_ULTRAHOT variable
   - Deleted front_prune_ultrahot_enabled() function
   - UltraHot feature was removed in commit bcfb4f6b5
   - Variable was dead code, no longer referenced

2. Organized ENV cleanup analysis documents
   - Moved 5 ENV analysis docs to docs/analysis/
   - ENV_CLEANUP_PLAN.md - detailed file-by-file plan
   - ENV_CLEANUP_SUMMARY.md - executive summary
   - ENV_CLEANUP_ANALYSIS.md - categorized analysis
   - ENV_CONSOLIDATION_PLAN.md - consolidation proposals
   - ENV_QUICK_REFERENCE.md - quick reference guide

Impact:
- ENV variables: 221 → 220 (-1)
- Build: ✅ Successful
- Risk: Zero (dead code removal)

Next steps (documented in ENV_CLEANUP_SUMMARY.md):
- 21 variables need verification (Ultra/HeapV2/BG/HotMag)
- SFC_DEBUG deduplication opportunity (7 callsites)

File: core/box/front_metrics_box.h
Status: SAVEPOINT - stable baseline for future ENV cleanup

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

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

core/box/front_metrics_box.h

@@ -133,21 +133,11 @@ static inline void front_metrics_class5_miss(int cls) {
 // ============================================================================
 // Purpose: Allow selective enabling/disabling of frontend layers
 // ENV Controls:
-//   HAKMEM_TINY_FRONT_ENABLE_ULTRAHOT=1  - Enable UltraHot magazine (C2-C5) [DEFAULT: OFF]
 //   HAKMEM_TINY_FRONT_DISABLE_HEAPV2=1   - Disable HeapV2 magazine (C0-C3) [DEFAULT: ON]
 //
-// Phase 19-4 A/B Test Result: UltraHot default OFF for +12.9% performance gain
+// Note: HAKMEM_TINY_FRONT_ENABLE_ULTRAHOT removed - UltraHot feature deleted in bcfb4f6b5
 // ============================================================================
 
-static inline int front_prune_ultrahot_enabled(void) {
-    static int cached = -1;
-    if (__builtin_expect(cached == -1, 0)) {
-        const char* env = getenv("HAKMEM_TINY_FRONT_ENABLE_ULTRAHOT");
-        cached = (env && *env && *env != '0') ? 1 : 0;  // DEFAULT: OFF (0) for best performance
-    }
-    return cached;
-}
-
 static inline int front_prune_heapv2_enabled(void) {
     static int cached = -1;
     if (__builtin_expect(cached == -1, 0)) {