Support graph indexing #77

Problem

zvec-grep currently relies primarily on text, FTS, and vector retrieval. These methods can find code that is textually or semantically similar to a query, but they do not preserve explicit relationships between code entities.

As a result, it is difficult to reliably answer structure-aware questions such as:

• Where is a symbol declared and defined?
• What does a function call, and which functions call it?
• Which types inherit from or implement a given type?
• Which symbols and files may be affected by a change?
• How are related symbols connected across files?

Expected behavior

zvec-grep should support graph indexing for code entities and their relationships, so graph-aware queries and exploration can complement the existing retrieval capabilities.

The implementation design and scope will be discussed separately.