Document Phase 4b master debug control in ENV_VARS.md

Add documentation for new HAKMEM_DEBUG_ALL and HAKMEM_DEBUG_LEVEL
environment variables introduced in Phase 4b.

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

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

docs/specs/ENV_VARS.md

@@ -336,7 +336,32 @@ ENV Statistics (from ENV_VARIABLE_SURVEY.md):
   - Free Path Optimization: 15 (7%)
   - Other: 23 (10%)
 
-Future Work (Phase 4b+):
+Phase 4b: Master Debug Control (COMPLETED)
-- HAKMEM_DEBUG unified variable (consolidate 35 debug vars)
+------------------------------------------
+New in 2025-11: Centralized debug control that works alongside individual module ENVs.
+
+- `HAKMEM_DEBUG_ALL=1`
+  - Enable ALL debug modules at once (convenient for troubleshooting)
+  - Individual module ENVs (e.g., HAKMEM_SFC_DEBUG=0) can still override
+
+- `HAKMEM_DEBUG_LEVEL=N`
+  - Set debug level: 0=off, 1=critical, 2=normal, 3=verbose
+  - When set to 2+, enables debug output for modules that don't have explicit ENV
+
+- `HAKMEM_QUIET=1`
+  - Suppress ALL debug output (highest priority, overrides DEBUG_ALL/LEVEL)
+
+Priority order:
+1. HAKMEM_QUIET=1 → suppress all
+2. Specific module ENV (e.g., HAKMEM_SFC_DEBUG=1) → use that value
+3. HAKMEM_DEBUG_ALL=1 → enable all
+4. HAKMEM_DEBUG_LEVEL >= threshold → enable
+5. Default → disabled
+
+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+):
 - HAKMEM_TRACE unified variable (consolidate 15 trace vars)
 - HAKMEM_STATS unified variable (consolidate 15 dump vars)