model: tensor-parallel Qwen3 (sharded weights + AllReduce)

from_weights_tp shards each rank's weights (column-split q/k/v/gate/up,
row-split o/down; replicate norms/embed/lm_head) and the paged forward uses
local head counts + AllReduces after o_proj and down_proj. PagedKVCache::new_tp
sizes the pool for the rank's local KV heads (KV is sharded too). TP=1 is the
identity path. New bench-tp binary runs E2E multi-GPU generation per TP degree.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/xserv-model/Cargo.toml

@@ -8,6 +8,7 @@ xserv-cuda = { path = "../xserv-cuda" }
 xserv-tensor = { path = "../xserv-tensor" }
 xserv-kernels = { path = "../xserv-kernels" }
 xserv-tokenizer = { path = "../xserv-tokenizer" }
+xserv-distributed = { path = "../xserv-distributed" }
 half.workspace = true
 smallvec.workspace = true
 serde.workspace = true