phase 3: GEMM kernels (naive, tiled, cuBLAS)

- Naive GEMM kernel: one thread per output element (F32 + BF16)
- Tiled GEMM kernel: 32x32 shared memory tiles (F32 + BF16)
- cuBLAS wrapper: cublasGemmEx with row-major trick
- GemmBackend enum for runtime backend selection
- CublasContext RAII handle
- Made error::check public for cross-crate use
- 17 GEMM tests: small/medium/rect sizes, all backends, F32+BF16
- Cross-backend consistency verified (naive vs tiled vs cuBLAS)
- All 44 tests pass across all crates

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/xserv-kernels/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "xserv-kernels"
+version.workspace = true
+edition.workspace = true
+
+[build-dependencies]
+cc = "1"
+
+[dependencies]
+xserv-cuda = { path = "../xserv-cuda" }
+xserv-tensor = { path = "../xserv-tensor" }
+half.workspace = true