Explicit links between notes in different notebooks are not shown

A note can be in one notebook and still have an explicit link pointing to a note that lives in a different notebook. That kind of link is created on
purpose and should always create a connection, but it does not show up when the two notes are in separate notebooks.

### Steps to reproduce
1. Create Note A in Notebook 1.
2. Create Note B in Notebook 2.
3. In Note A, add a link pointing to Note B.
4. Open the Note Graph panel.

### Expected behavior
A connection appears between Note A and Note B, since the link between them is explicit and intentionally authored.

### Actual behavior
No connection appears. Links only appear to be built between notes that share the same notebook, so cross-notebook references are missing from the graph.

Start at the Note Graph panel and trace how explicit links are collected when notes belong to different notebooks. Reproduce the issue with two notebooks and inspect the link-building path for any same-notebook restriction. Done means an intentionally authored link between Note A and Note B appears in the graph across notebooks.

Written by the indexing model from the issue text.