Expose bundle-local Needs inventories as JSON file artifacts #898

Problem

When external_needs names a public docs() or docs_bundle target, the macro resolves it to the private .__internal__.needs_local target. The Sphinx action declares that target as a directory artifact at _build/needs, while the runtime code finds the inventory by appending _build/needs/needs.json to the target's runfiles path.

• The Sphinx action declares a directory output
• The runtime reconstructs the JSON path inside that directory

This works, but the private target represents a whole Sphinx output directory even though external loading and duplicate-ID preflight consume only needs.json. The runtime also depends on the internal Sphinx output layout.

Desired outcome

Expose the bundle-local inventory through a private Bazel target whose artifact is the needs.json file, and have external_needs resolve public bundle labels to that file artifact. Keep the public external_needs API unchanged.

Update the runtime path resolution and duplicate-ID preflight to consume the file artifact directly. Preserve any directory output still needed by the Sphinx builder or existing targets such as :needs_json and :metrics_json; choose the smallest Bazel-native implementation that provides the direct file target.

Related work

#806 covers resolving inventory artifacts when documentation targets are renamed. This issue focuses on the bundle-local inventory artifact itself being a directory rather than a JSON file.