Stage 7b-ii: ColumnParallel + RowParallel sharded linear primitives

Adds harness/tp/sharded_linear.rs with ShardedLinear — a Megatron-LM
style sharded wrapper over candle_nn::Linear. Two constructors:

- load_column: splits the output dimension. Each rank holds rows
  [r*out/N .. (r+1)*out/N] of the weight, plus its slice of the bias.
  Forward = local matmul; output is naturally sharded; downstream
  consumer either accepts the shard (next layer is column-parallel)
  or merges via all-gather later.
- load_row: splits the input dimension. Each rank holds cols
  [r*in/N .. (r+1)*in/N] of the weight; bias lives only on rank 0
  so the post-all_reduce sum carries it exactly once. Forward
  produces a partial output that the caller reduces via NCCL.

Both constructors bail with a clear error when divisibility doesn't
hold — the precondition mistral.rs's first qwen3-next-tp commit
made explicit. The path included in the error is the VarBuilder
prefix, so the operator sees exactly which projection failed
("column-parallel 'model.layers.0.self_attn.q_proj': out_features=...").

5 unit tests on CPU verify the math against an unsharded reference:
- column shard produces the expected slice of the full matmul
- row partials sum to the unsharded result
- row bias appears only on rank 0
- divisibility violations bail (column + row)

forward_with_comm() is stubbed for row-parallel (CUDA-only) — wiring
the actual cudarc::nccl all_reduce against candle's Tensor lands in
7b-iii alongside the model assembly, where the model holds the Comm
in scope. ColumnParallel's forward_with_comm just delegates to the
local matmul (no collective needed).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/neuron/src/harness/tp/mod.rs

@@ -19,6 +19,7 @@
 
 pub mod nccl_state;
 pub mod rpc;
+pub mod sharded_linear;
 pub mod worker;
 
 use anyhow::{Context, Result};