Feat: Add experimental TLS Bind Box path in Unified Cache

- Added experimental path in unified_cache_refill to test ss_tls_bind_one for C7 class.
- Guarded by HAKMEM_WARM_TLS_BIND_C7 env var and debug build.
- Updated Page Box comments to clarify future TLS Bind Box integration.

core/box/tiny_page_box.h

@@ -13,11 +13,12 @@
 //       `max` BASE pointers using per-page freelist before falling back.
 //   - When disabled for a class: the box returns 0 and caller uses legacy path.
 //
-//   - TLS Bind:
-//       Future direction: The Page Box will select a (SuperSlab, slab_idx)
-//       pair and use ss_tls_bind_one() to bind it to TLS. Subsequent
-//       allocations will carve directly from that TLS-bound slab,
-//       clarifying the boundary between Superslab Backend and TLS Bind.
+//   - TLS Bind Responsibility:
+//       Page Box selects the appropriate (SuperSlab, slab_idx) pair for the
+//       current request (prioritizing EMPTY or HOT slabs).
+//       It then delegates the binding operation to ss_tls_bind_one() (TLS Bind Box).
+//       This separates "Resource Selection" (Page Box) from "Context Binding"
+//       (TLS Bind Box), clarifying the boundary with Superslab Backend.
 //
 // ENV:
 //   HAKMEM_TINY_PAGE_BOX_CLASSES (optional)