Make ⊗ the Kronecker product

[isomorphisms]

Make ⊗ carry its intended mathematical meaning instead of using it as an arbitrary Unicode-infix placeholder.

• defines ⊗ as the same binary operation as kronecker(X, Y) / %x%
• adds a real 2×2 ⊗ 2×2 acceptance case with an explicit expected 4×4 matrix
• adds a column-vector tensor-product example
• keeps the generic-Unicode-infix lexer test, but moves that placeholder role to ⊙
• documents the distinction that R's %x% is the Kronecker product while %o% is the outer product
• runs the top-level tensor-product example in CI

This is stacked on the equality-glyph work because it extends the same Unicode lexer regression.