Phase 70: Defined observability prerequisites SSOT

- Added docs/analysis/PHASE70_REFILL_OBSERVABILITY_PREREQS_SSOT.md to clarify that refill/warmpool optimizations require confirmed cache misses to be measurable.
- Updated CURRENT_TASK.md to point to this prerequisite.

docs/analysis/PHASE70_REFILL_OBSERVABILITY_PREREQS_SSOT.md

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+# Phase 70: Refill Tuning Prerequisites (Observability SSOT)
+
+**Status**: 🟡 ACTIVE
+
+**Context**: 
+- Current baseline (Mixed WS=400 + prefault) generates almost **zero cache misses** (refills) in Unified Cache.
+- Optimizing `unified_cache_refill()` or Warm Pool logic will yield **zero throughput gain** if this path is not hot.
+- Phase 70 must be gated on *observing significant refill activity*.
+
+## 1. Observability Protocol (Step 0)
+
+Before implementing any refill/WarmPool changes, execute this sequence:
+
+1.  **Build with Stats**:
+    ```bash
+    make bench_random_mixed_hakmem_observe EXTRA_CFLAGS='-DHAKMEM_UNIFIED_CACHE_STATS_COMPILED=1'
+    ```
+    *(Note: Phase 70-0 fixed release-mode stats blocking)*
+
+2.  **Run with Stats**:
+    ```bash
+    HAKMEM_WARM_POOL_STATS=1 ./bench_random_mixed_hakmem_observe 20000000 400 1
+    ```
+
+3.  **Check Output**:
+    - Look for `Unified-STATS`: `miss=...`
+    - Look for `WarmPool-STATS`: `hits=...`
+
+**Decision Gate**:
+- If `miss` counts are < 1000 (approx <0.01% miss rate):
+  - **STOP**: Optimization has no ROI on this workload.
+  - **ACTION**: Either accept current state (refill is not a bottleneck) or switch to a research workload (below).
+- If `miss` counts are significant:
+  - **GO**: Proceed with Phase 70 logic changes.
+
+## 2. Research Workload (If Refill Optimization is Mandatory)
+
+If you must measure refill performance improvements (e.g., for architectural validation), modify the workload to force cache pressure:
+
+**Option A: Disable Prefault (Cold Path Stress)**
+```bash
+HAKMEM_BENCH_PREFAULT=0 ./bench_random_mixed_hakmem_observe ...
+```
+- Pros: Forces refills during benchmark loop.
+- Cons: Measures startup behavior, not steady-state throughput.
+
+**Option B: Increase Working Set (Steady State Stress)**
+```bash
+./bench_random_mixed_hakmem_observe 20000000 8192 1  # WS=8192 > Cache(2048)
+```
+- Pros: Forces steady-state evictions/refills.
+- Cons: Different workload profile than standard Mixed SSOT (WS=400).
+
+**WARNING**: Do NOT change `HAKMEM_TINY_STATIC_ROUTE` or `ULTRA` flags unless specifically testing routing changes. The default `LEGACY` route *does* use Unified Cache for alloc/free.
+
+## 3. Reference: Why "LEGACY" Route is OK
+
+- **LEGACY** route means "not ULTRA/MID/V7 specialized".
+- Alloc path: `malloc_tiny_fast` → `tiny_hot_alloc_fast` → **Unified Cache (TLS array)**.
+- Free path: `free_tiny_fast` → `tiny_hot_free_fast` → **Unified Cache (TLS array)**.
+- Refill path: Cache Miss → `unified_cache_refill` → **Warm Pool** → Registry.
+
+Previous confusion ("LEGACY unused") was due to:
+1. Stats counting was gated by `#if !HAKMEM_BUILD_RELEASE`.
+2. Low miss rate in WS=400 made it look unused.
+3. Phase 70-0 fixed the stats visibility.