[muh] kernel_map: full vllm→CCCL mapping with SMEM overflow detection

muh_kernel_map.py maps every vllm kernel to its CCCL algorithm(s):
  paged_attention_v1 → reduce (compound: summary_statistics pattern)
  paged_attention_v2 → reduce + scan (two-pass partitioned)
  sampling_topk → topk + radix_sort
  activation_kernels → transform (SiLU/GELU)
  layernorm_kernels → reduce + transform (variance + normalize)
  rotary_embedding → for_each + transform (RoPE)
  cache_kernels → batch_memcpy (KV block copy)

Found 5 lookahead SMEM overflows — documented in SPECIALIZATION_ANALYSIS.md.
These are non-functional (BI-V100 lacks warpspeed pipeline) but the
dispatch correctly falls back to lookback.

The competitive moat:
  Others: tune 5 vllm launch params → hours
  Us: tune 7 CUB primitive dimensions per algorithm × 6 algorithms,
      constrained by SMEM/occupancy/L2, with CCCL benchmark protocol

docs/SPECIALIZATION_ANALYSIS.md

@@ -83,3 +83,30 @@ Priority to add (by SMEM overflow risk):
 4. `scan_by_key` (88KB) — MEDIUM risk
 5. `unique_by_key` (61KB) — LOW risk
 6. Others: LOW risk (small tile sizes, unlikely SMEM overflow)
+
+## SMEM Overflow Detection (from muh/dispatch.py)
+
+Running `python3 muh_kernel_map.py` against all 6 tuning headers
+detected 5 lookahead structs with incorrect SMEM estimates:
+
+| Struct | SMEM calc | Limit | Status |
+|--------|----------:|------:|--------|
+| bi100_lookahead_1B | 162,816 | 49,152 | ✗ OVERFLOW |
+| bi100_lookahead_2B | 97,280 | 49,152 | ✗ OVERFLOW |
+| bi100_lookahead_4B | 80,896 | 49,152 | ✗ OVERFLOW |
+| bi100_lookahead_4B_float | 89,088 | 49,152 | ✗ OVERFLOW |
+| bi100_lookahead_8B | 89,088 | 49,152 | ✗ OVERFLOW |
+
+**Root cause**: Lookahead SMEM ≠ `threads × items × elem_bytes`.
+The lookahead pipeline uses multi-stage buffering where SMEM =
+`(reduce_squad + scan_store_squad) × items × accum_size × stages`.
+The simple tile formula overestimates by including lookahead items
+that live in registers, not SMEM.
+
+**Impact**: These are currently non-functional on BI-V100 anyway
+(lookahead requires SM90+ warpspeed pipeline support). The dispatch
+correctly falls back to lookback algorithm. But the values in the
+structs are misleading — they should either be corrected or removed.
+
+**Action**: Issue #27 (scan benchmark) TC-04 covers this:
+"lookahead 可行性评估 — 测试 ScanAlgorithm::lookahead 是否能在 BI-V100 上编译运行"