autodiff: finite-diff gradient-check harness

New xtrain-autodiff crate with a reusable central finite-difference
gradient check: grad_check(x, shape, f, analytic_grad, cfg) compares an
analytic gradient against (f(x+ε)-f(x-ε))/2ε per element with a relative
tolerance. Host-only (no CUDA): the loss closure owns any GPU work, so
T4's per-op backward checks can reuse it directly. Includes host unit
tests (sum(x²) grad 2x passes; a wrong grad is rejected).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Cargo.toml

@@ -3,6 +3,7 @@ resolver = "2"
 members = [
     "crates/xtrain-cuda",
     "crates/xtrain-tensor",
+    "crates/xtrain-autodiff",
 ]
 
 [workspace.package]