[PR4] Specify cross-bundle Needs propagation

[AlexanderLanin]

Why this matters

A bundle-local Needs export can validate and publish the Needs defined by that bundle, but composed documentation hierarchies also need a deliberate way to resolve references to Needs owned by related bundles. The propagation rules need to be explicit before implementation: which ancestors are included, whether inheritance is transitive, and what happens when a bundle has no own sources.

What changed

• Add a rough upward_bundles fixture describing direct-ancestor propagation.
• Specify that local exports remain limited to a bundle’s own sources.
• Specify non-transitive inheritance and rejection of source-less ancestors.
• Keep data-only bundles outside the Needs export hierarchy.

This is intentionally a rough specification draft. The implementation and final API will follow after the behavior is agreed.