Document Phase 4c/4d master trace and stats control

Complete ENV cleanup Phase 4 documentation:
- Phase 4c: HAKMEM_TRACE unified trace control
- Phase 4d: HAKMEM_STATS unified stats control
- Summary of all 6 new master control variables

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

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

docs/specs/ENV_VARS.md

@@ -362,6 +362,55 @@ Implementation: core/hakmem_debug_master.h
 - hak_debug_check("HAKMEM_FOO_DEBUG") - Check if module should enable debug
 - hak_is_quiet() - Quick check for quiet mode
 
-Future Work (Phase 4c+):
+Phase 4c: Master Trace Control (COMPLETED)
-- HAKMEM_TRACE unified variable (consolidate 15 trace vars)
+------------------------------------------
-- HAKMEM_STATS unified variable (consolidate 15 dump vars)
+New in 2025-11: Unified trace control.
+
+- `HAKMEM_TRACE=all`
+  - Enable ALL trace modules at once
+
+- `HAKMEM_TRACE=ptr,refill,free,mailbox`
+  - Enable specific trace modules (comma-separated)
+
+- `HAKMEM_TRACE_LEVEL=N`
+  - Set trace verbosity (1=basic, 2=detailed, 3=verbose)
+
+Available trace modules:
+  ptr, refill, superslab, ring, free, mailbox, registry
+
+Implementation: core/hakmem_trace_master.h
+
+Phase 4d: Master Stats Control (COMPLETED)
+------------------------------------------
+New in 2025-11: Unified stats/dump control.
+
+- `HAKMEM_STATS=all`
+  - Enable ALL stats modules at once
+
+- `HAKMEM_STATS=sfc,fast,pool`
+  - Enable specific stats modules (comma-separated)
+
+- `HAKMEM_STATS_DUMP=1`
+  - Dump stats at process exit
+
+Available stats modules:
+  sfc, fast, heap, refill, counters, ring, invariant,
+  pagefault, front, pool, slim, guard, nearempty
+
+Implementation: core/hakmem_stats_master.h
+
+---
+
+ENV Cleanup Summary (2025-11)
+=============================
+- Phase 4a: Hot path getenv caching (hakmem_elo.c fixed)
+- Phase 4b: HAKMEM_DEBUG_ALL/LEVEL master debug control
+- Phase 4c: HAKMEM_TRACE unified trace control
+- Phase 4d: HAKMEM_STATS unified stats control
+
+Total new master control variables: 6
+  HAKMEM_DEBUG_ALL, HAKMEM_DEBUG_LEVEL
+  HAKMEM_TRACE, HAKMEM_TRACE_LEVEL
+  HAKMEM_STATS, HAKMEM_STATS_DUMP
+
+All existing individual ENVs continue to work (backwards compatible)