v0.18.0 — parse Relationship from Cube joins config

Added

• Cube schema relationship parsing: CubeSource.get_relationships() and get_relationships_for_table() no longer return empty stubs. Each cube's joins: block is parsed and projected into Relationship instances, completing the trifecta with YamlSource (v0.16.0) and DbtSource (v0.17.0) — every built-in semantic source now carries the same preferred, required_filter, and relationship_type semantics end-to-end.
• from/to normalisation: The Relationship's from is always the column on the cube declaring the join, regardless of which side {CUBE} appears on in the SQL. Authors who write {CUBE}.fk = {Other}.pk and authors who write {Other}.pk = {CUBE}.fk get identical Relationship instances. The type carries the cardinality (many_to_one / one_to_one / one_to_many), so a hasMany join on cube A produces A.pk -> B.fk with type=one_to_many, matching how YamlSource authors hand-write the same pattern.
• Cube enum mapping: belongsTo / hasOne / hasMany (camelCase v1) and many_to_one / one_to_one / one_to_many (snake_case v2 aliases) all collapse to the canonical Relationship.type strings via a small lookup table. meta.relationship_type overrides for unusual cases.
• meta: block convention matches DbtSource's v0.17.0 contract: meta.preferred, meta.required_filter, meta.relationship_type on each join entry.
• Index reuse via build_relationship_index — CubeSource inherits the v0.16.0 preferred-first stable-sort guarantee, same as YamlSource and DbtSource.

Documentation

• 11 new tests in tests/test_semantic/test_cube.py (TestCubeRelationships) covering: load count with one phantom-cube reference filtered, canonical edge round-trip, non-preferred default, reversed-SQL parsing ({Other}.pk = {CUBE}.fk form), hasMany -> one_to_many alias mapping, self-FK with meta.relationship_type overriding hasOne, preferred-first index ordering, referenced-side indexing, self-FK appearing once not twice, the original empty-joins fixture continuing to return [], and unresolvable cube names being silently skipped.
• New fixture tests/fixtures/sample_cube_schema_with_joins.yml — Orders cube with two role-playing joins into Users (customer_id preferred, sales_rep_id not), a Customers cube with a hasMany join into Orders, a self-referencing Employees cube, and one deliberately broken join pointing at a non-existent cube to exercise the silent-skip path. Existing sample_cube_schema.yml left untouched as the empty-joins baseline.
• README's Semantic Sources / Cube section documents the parser convention with a joins: example showing all three meta: keys; architecture doc gains a paragraph mirroring the v0.17.0 dbt note.

Limitations

• Composite-key joins ({CUBE}.tenant_id = {Other}.tenant_id AND {CUBE}.id = {Other}.parent_id) are not parsed — declare those via YamlSource instead. The TODO surfaces as a "skipped silently" outcome rather than an error so existing Cube schemas don't break.