#404c Parse tree-sitter once per file #412

Problem

When tree-sitter feature is enabled, ast::extract() is called three separate times for the same file:

1. extract_symbols() -- discards edges
2. extract_calls() -- discards nodes, re-parses
3. extract_edges() -- re-parses again

Each call re-parses the entire source into a new tree-sitter tree.

Location

src/index/extract.rs:198, 513, 595

Solution

Parse once in index_file(), pass (nodes, edges) to all three consumers.

Expected Impact

2-3x reduction in parsing cost for tree-sitter path.