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)

prefix_prefill.py

@@ -726,12 +726,8 @@ if triton.__version__ >= "2.1.0":
         #   Triton equivalent: BLOCK=64, warps=4 (128 threads, larger tile per warp)
         _is_bi_v100 = not current_platform.has_device_capability(80)
         if _is_bi_v100:
-            # muh: CCCL-informed block selection for BI-V100 (SM=16, SMEM≤48KB)
-            # SMEM = BLOCK_M*Hd*elem + BLOCK_N*Hd*elem*2(K+V)
-            # head_dim=128, fp16(2B): BLOCK=64,N=64 → 48KB (100% SMEM, risky)
-            # Conservative: BLOCK=64,N=32 → 32KB (65% SMEM, safe for 32KB limit)
             BLOCK = 64
-            NUM_WARPS = 4   # 4 warps × 32 = 128 threads; BW-limited at 56 GB/s/SM
+            NUM_WARPS = 4
         else:
             BLOCK = 128
             NUM_WARPS = 8