optim: hand-written AdamW (decoupled weight decay + bias correction)

New xtrain-optim crate. AdamW with per-param m/v moments keyed by params()
index, global bias correction, and decoupled weight decay (matches
torch.optim.AdamW). Split into a pure-host step_host (flat f32 buffers,
unit-testable on a GPU-less host) and a step(&[Var]) wrapper that round-trips
each param value/grad through the GPU tensor (gated not(no_cuda)). Per-step lr
argument leaves room for an LR schedule.

Host unit test checks the update against an independent reference recurrence
over 20 steps and the pure-decay (g=0) boundary.

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

crates/xtrain-optim/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "xtrain-optim"
+version.workspace = true
+edition.workspace = true
+
+[dependencies]
+xtrain-tensor = { path = "../xtrain-tensor" }
+xtrain-autodiff = { path = "../xtrain-autodiff" }
+
+[dev-dependencies]
+# Acceptance tests drive the GPU (device selection) directly.
+xtrain-cuda = { path = "../xtrain-cuda" }