Multi-graph support with GraphManager and FROM clause

[dylanbstorey]

Summary

• Adds complete multi-graph functionality for querying across multiple graph databases using SQLite ATTACH
• Implements FROM clause in MATCH patterns (e.g., MATCH (n:Person) FROM social)
• Adds graph() function to identify which graph a node comes from
• Creates GraphManager class/struct for Python and Rust bindings

Features

Core

• FROM clause support in MATCH patterns
• Table prefixing for cross-graph property access
• Variable tracking with graph association
• graph() function for node source identification

Python Bindings

• GraphManager class with list(), create(), open(), drop(), query(), query_sql()
• graphs() factory function with context manager support
• 22 tests covering all functionality

Rust Bindings

• GraphManager struct with similar API
• New error types: GraphExists, GraphNotFound
• Graph::open_with_extension() method
• 7 unit tests

Documentation

• New how-to guide: "Working with Multiple Graphs"
• Updated Python and Rust API references

Example Usage

from graphqlite import graphs

with graphs("./data") as gm:
    social = gm.create("social")
    social.upsert_node("alice", {"name": "Alice"}, "Person")
    
    # Cross-graph query
    result = gm.query(
        "MATCH (n:Person) FROM social RETURN n.name, graph(n) AS source",
        graphs=["social"]
    )

Test plan

• Unit tests pass (753 C tests)
• Python GraphManager tests pass (22 tests)
• Rust GraphManager tests pass (7 tests)
• Functional multi-graph tests pass