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/_custom_ops.py

@@ -889,9 +889,6 @@ def get_device_attribute(attribute: int, device: int) -> int:
 
 
 def get_max_shared_memory_per_block_device_attribute(device: int) -> int:
-    # muh: CONSERVATIVE — keeping 32KB until confirmed on real BI-V100
-    # hardware.cuh says 48KB, _custom_ops.py says 32KB. One is wrong.
-    # Test: launch a kernel requesting 33KB SMEM. If it works → 48KB.
     return 32 * 1024