ENV Cleanup Phase 5: Additional DEBUG guards + doc cleanup

Code changes:
- core/slab_handle.h: Add RELEASE guard for HAKMEM_TINY_FREELIST_MASK
- core/tiny_superslab_free.inc.h: Add guards for HAKMEM_TINY_ROUTE_FREE, HAKMEM_TINY_FREELIST_MASK

Documentation cleanup:
- docs/specs/CONFIGURATION.md: Remove 21 doc-only ENV variables
- docs/specs/ENV_VARS.md: Remove doc-only variables

Testing:
- Build: PASS (305KB binary, unchanged)
- Sanity: PASS (17.22M ops/s average, 3 runs)
- Larson: PASS (52.12M ops/s, 0 crashes)

Impact:
- 2 additional DEBUG ENV variables guarded (no overhead in RELEASE)
- Documentation accuracy improved
- Binary size maintained

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

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

core/slab_handle.h

@@ -260,10 +260,14 @@ static inline int slab_freelist_push(SlabHandle* h, void* ptr) {
    // Optional freelist mask update (opt-in via env HAKMEM_TINY_FREELIST_MASK)
    do {
        static int g_mask_en = -1;
+#if HAKMEM_BUILD_RELEASE
+       g_mask_en = 0;
+#else
        if (__builtin_expect(g_mask_en == -1, 0)) {
            const char* e = getenv("HAKMEM_TINY_FREELIST_MASK");
            g_mask_en = (e && *e && *e != '0') ? 1 : 0;
        }
+#endif
        if (__builtin_expect(g_mask_en, 0) && prev == NULL && h->ss) {
            uint32_t bit = (1u << h->slab_idx);
            atomic_fetch_or_explicit(&h->ss->freelist_mask, bit, memory_order_release);
@@ -310,10 +314,14 @@ static inline void* slab_freelist_pop(SlabHandle* h) {
         // Optional freelist mask clear when freelist becomes empty
         do {
             static int g_mask_en2 = -1;
+#if HAKMEM_BUILD_RELEASE
+            g_mask_en2 = 0;
+#else
             if (__builtin_expect(g_mask_en2 == -1, 0)) {
                 const char* e = getenv("HAKMEM_TINY_FREELIST_MASK");
                 g_mask_en2 = (e && *e && *e != '0') ? 1 : 0;
             }
+#endif
             if (__builtin_expect(g_mask_en2, 0) && next == NULL && h->ss) {
                 uint32_t bit = (1u << h->slab_idx);
                 atomic_fetch_and_explicit(&h->ss->freelist_mask, ~bit, memory_order_release);