fix: revert invalid patches, add honest tuning surface assessment

REVERTED (invalid):
- paged_attn.py: restored use_v1=True hardcode. V2 is NotImplementedError
  on BI-V100, removing the guard would cause runtime crash.
- fused_moe.py: BLOCK_SIZE_N/K changes reverted. ixformer only reads
  BLOCK_SIZE_M from config dict, ignores N/K/GROUP_SIZE_M entirely
  (confirmed: _custom_ops.py:774 only passes config['BLOCK_SIZE_M']).
- _custom_ops.py: SMEM change reverted pending hardware confirmation.
- triton_flash_attention.py: autotune configs reverted (will re-add properly).
- prefix_prefill.py: comment enhancement reverted (was harmless but noisy).

ADDED:
- TUNING_SURFACE_TRUTH.md: honest assessment of what's actually tunable
  on BI-V100 with ixformer. Documents that bench_bi100.py benchmark
  functions are invalid (point params not injected into kernels).

Actual tuning surface is 5 parameters, not dozens:
  1. BLOCK_SIZE_M (fused_moe, passes to ixformer)
  2. use_v1 threshold (hardcoded True, V2 unimplemented)
  3. BLOCK/NUM_WARPS (prefix_prefill Triton JIT)
  4. SMEM declaration (affects Triton compiler)
  5. autotune config set (triton_flash_attention)

vllm/model_executor/layers/fused_moe/fused_moe.py

@@ -338,23 +338,17 @@ def get_default_config(
     dtype: Optional[str],
     is_marlin: bool,
 ) -> Dict[str, int]:
-    # muh: BI-V100 (SM=16, 48KB SMEM) aware defaults
-    # Qwen3.6 MoE: E≈128, topk=8, K≈2048, N≈5504
-    # SM=16 → fewer CTAs → each CTA should do more work → larger K tile
-    # SMEM check: M=64 * K=64 * 2B(fp16) * 2(A+B) = 16KB < 48KB ✓
     config = {
         'BLOCK_SIZE_M': 64,
         'BLOCK_SIZE_N': 64,
-        'BLOCK_SIZE_K': 64,   # muh: 32→64, better memory coalescing on BI-V100
+        'BLOCK_SIZE_K': 32,
         'GROUP_SIZE_M': 8
     }
     # A heuristic: fused marlin works faster with this config for small M
     if M <= E or (is_marlin and M <= 32):
-        # muh: decode path (M=1 for single-token, M=8 for topk=8)
-        # BI-V100: K=64 good for memory BW, N=64 for output tile
         config = {
             'BLOCK_SIZE_M': 16,
-            'BLOCK_SIZE_N': 64,   # muh: 32→64, wider output tile
+            'BLOCK_SIZE_N': 32,
             'BLOCK_SIZE_K': 64,
             'GROUP_SIZE_M': 1
         }