refactor: template AttentionParams, rename .cuh to .h

- Convert AttentionParams to a template struct supporting arbitrary types
- Rename attn_common.cuh -> attn_common.h (no CUDA-specific code remains)
- Include standard headers explicitly in each .cuh instead of via attn_common.cuh
- Allow .h files in csrc/ via .gitignore

csrc/tests/attn_prefill_test.cu

@@ -23,7 +23,7 @@ static double now_ms() {
 // Launch the production prefill path (tensor-core MMA on sm_80+, else the
 // scalar fallback), mirroring dispatch_prefill() in attn_prefill.cu.
 template <int HEAD_DIM>
-static void launch_prefill(AttentionParams& p) {
+static void launch_prefill(AttentionParams<bf16>& p) {
 #ifndef ASTRAI_NO_MMA
     constexpr int WARPS = 4, BR = 16;
     constexpr int BC = (HEAD_DIM <= 128) ? 32 : 16;
@@ -40,7 +40,7 @@ static void launch_prefill(AttentionParams& p) {
 #endif
 }
 
-static void dispatch_prefill(AttentionParams& p) {
+static void dispatch_prefill(AttentionParams<bf16>& p) {
     switch (p.head_dim) {
         case 64:  launch_prefill<64>(p);  break;
         case 128: launch_prefill<128>(p); break;
@@ -123,7 +123,7 @@ static void bench() {
         for (size_t i=0;i<nKV;i++) tmp[i]=f2bf(randf());
         cudaMemcpy(dV,tmp,nKV*2,cudaMemcpyHostToDevice);
 
-        AttentionParams p;
+        AttentionParams<bf16> p;
         p.batch=B; p.q_head=Hq; p.kv_head=Hk; p.q_len=ql; p.kv_len=kl; p.head_dim=D;
         p.use_mask=0; p.is_causal=causal; p.causal_offset=0;
         p.scale=1.0f/sqrtf((float)D);
@@ -191,7 +191,7 @@ int main() {
         for (size_t i=0;i<nKV;i++) tmp[i]=f2bf(hV[i]);
         cudaMemcpy(dV,tmp,nKV*2,cudaMemcpyHostToDevice);
 
-        AttentionParams p;
+        AttentionParams<bf16> p;
         p.batch=B; p.q_head=Hq; p.kv_head=Hk; p.q_len=ql; p.kv_len=kl; p.head_dim=D;
         p.use_mask=0; p.is_causal=causal; p.causal_offset=0;
         p.scale=1.0f/sqrtf((float)D);