Add sparse tensor primitives with einsum contraction

[imlvts]

Integrate N-dimensional sparse tensors into MORK expressions:

• SparseTensorF64: PathMap-backed sparse tensor with BOB encoding, arbitrary rank, element-wise add/mul via lattice operations
• einsum-dyn: copied into workspace as local crate, runtime Einstein summation with VM compiler supporting arbitrary N-D specs
• Tensor sinks (tensor_collect, tensor_einsum, tensor_add, tensor_mul, tensor_free) using WriteResource::TensorStore through the standard resource infrastructure — no thread-locals
• tensor_get/tensor_nnz pure functions via ExprSource.context pointer propagated through EvalScope
• End-to-end test: load matrix data as S-expressions, collect into tensors via exec, einsum multiply, verify result

MeTTa usage:

(exec P1 (, (a $r $c $v)) (O (tensor_collect A $r $c $v)))
(exec P2 (, (b $r $c $v)) (O (tensor_collect B $r $c $v)))
(exec P3 (,) (O (tensor_einsum "ab,bc->ac" A B C)))