Add ignored regression test for optional full-outer-join scalar access kind

[cmontella]

Motivation

• Capture a regression where scalar access of an optional table column produced Reference(Empty) instead of an Option element kind after a full outer join (i.e. x.hw1[4] should have kind u8?).

Description

• Add a focused interpreter test interpret_table_full_outer_join_optional_column_scalar_access_kind_is_u8_option to tests/interpreter.rs that exercises x := a ⟗ b; y := x.hw1[4]; and asserts the expected Option(U8) kind.
• Mark the new test #[ignore] with the reason: "Known limitation: scalar access from MatrixValue loses optional element kind and returns Reference(Empty)." to document the current behavior without changing runtime semantics.
• Keep existing tests for the matrix-level kind (x.hw1) and unwrap behaviors intact.

Testing

• Ran the matrix-kind test interpret_table_full_outer_join_optional_column_kind_is_u8_option_matrix, which passed.
• Ran the new scalar-kind test once which failed (demonstrating the issue), then marked it #[ignore].
• Ran the grouped test filter cargo test interpret_table_full_outer_join_optional_column -- --nocapture which resulted in 3 passed, 0 failed, 1 ignored (the newly added ignored test).

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Codex Task