distributed: process-per-GPU launcher + worker (proc.rs)

torchrun-style process-per-GPU: launch_processes spawns one worker process per
GPU (re-exec current_exe with XTRAIN_{RANK,WORLD,LOCAL_RANK,NCCL_ID} env),
mints the ncclUniqueId once in the launcher and hex-injects it via env (no
shared FS/TCP, race-free). worker_env/run_worker read the env, bind the device
(own CUDA context), DdpContext::init + build_model + train_rank reused from T8
UNCHANGED. hex_encode/decode_unique_id are host-testable pure fns.

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

crates/xtrain-distributed/src/lib.rs

@@ -18,8 +18,13 @@
 
 pub mod ddp;
 pub mod ffi;
+pub mod proc;
 
 pub use ddp::{DdpConfig, DdpResult, build_model, launch, train_rank};
+pub use proc::{
+    ModelOpts, WorkerEnv, build_worker_model, hex_decode_unique_id, hex_encode_unique_id,
+    launch_processes, run_worker, worker_env,
+};
 
 use std::ffi::c_void;