Experiment: Fused Q·Centroid compressed attention for turbo3 decode

## Problem turbo3 decode has 82M data-dependent constant memory accesses per token (centroid LUT lookup). On M5 Max this costs ~9% speed. On M1 Max it causes a cliff from 84% to 39% of q8_0 as context grows — L2 cache pressure evicts centroid entries to main memory. ## Current decode ratio curve (turbo3/q8_0) | Depth | M5 Max | M1 Max | |-------|--------|--------| | short | 0.91x | 0.84x | | 4K | 0.89x | ~0.64x | | 8K | 0.86x | ~0.54x | | 16K | — | ~0.45x | | 32K | — | ~0.39x | ## The Fix Eliminate constant memory from the FA decode inner loop. Three approaches to try: ### Approach 1: half cn[8] registers (16 bytes, may not spill) Previous float cn[8] (32 bytes) spilled on Metal. Half-precision halves register pressure. ### Approach 2: Threadgroup centroid cache Load 8 centroids to threadgroup memory once per threadgroup. Previous test was invalid (CPU fallback bug). Never tested on real Metal GPU. ### Approach 3: Per-block norm*centroid table Precompute `cn_norm[c] = centroid[c] * norm` at block start. Inner loop becomes `score += cn_norm[idx] * q[j]`. Fresh 8-entry register array per block, maximally cache-friendly. ## Success criteria - turbo3/q8_0 decode ratio stays FLAT across context depths (currently drops 0.91x to 0.72x on M5) - If flat at 0.90x+ across all depths, the fix works - PPL unchanged (dequant values identical, just accessed differently) ## Prior work - buun's register LUT: 0.965x on CUDA, spilled to 0.879x on Metal (float[8]) - Split-LUT (2x4 half...