Mark outer-join columns optional and add optimized matrix/table access handlers

[cmontella]

Motivation

• Make table outer-join semantics preserve sparsity by marking columns that may be absent as optional types for downstream consumers.
• Add direct/runtime/compile support for accessing scalar values out of MatrixValue and optional table columns to simplify and optimize access paths.

Description

• Update TableJoinFxn::build_joined_table to mark output column ValueKind as Option(...) for columns coming from the non-preserved side of outer joins and add helper make_optional_kind.
• Add a fast-path MatrixAccessScalarValueF implementation with solve, out, to_string and a compiler implementation and use it in the MatrixAccessScalar compiler when the LHS is a Value::MatrixValue and the index is a Value::Index.
• Add a special-case in impl_access_column_table_fxn to return a swizzle when the requested table column is an Option value to enable direct optional-column access.
• Add interpreter tests exercising full-outer-join behavior and optional column access, and update existing access code to incorporate the new behaviors.

Testing

• Ran the interpreter test suite with the new tests under tests/interpreter.rs including interpret_table_full_outer_join_marks_sparse_columns_optional and the optional-column access cases, and they passed.
• Ran the table join related tests such as interpret_table_full_outer_join_symbol/interpret_table_full_outer_join_word and the left/right/inner join tests, and they passed.
• Ran relevant matrix access and reshape interpreter tests referenced in tests/interpreter.rs and they passed.

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Codex Task