Class.lines always 0 for all languages

Discovered while trying to sort classes by size. Every class node had lines = 0 regardless of actual length.

Root cause

In extract_class_def(), start_line and end_line are correctly set from tree-sitter node positions, but def.lines is never computed — it stays 0 from the memset that zeroes the struct. The equivalent computation exists and is correct in push_method_def():

// push_method_def — correct
def.lines = (int)(def.end_line - def.start_line + TS_LINE_OFFSET);

// extract_class_def — missing this line entirely
// def.lines was always 0

Fix

Add the same one-liner to extract_class_def() after end_line is set. Affects all languages — any tree-sitter grammar that produces class nodes goes through this path.

Regression test

tool_qg_class_lines_nonzero in tests/test_incremental.c — indexes a project in full mode and asserts that at least one class node has lines > 0 via a query_graph tool call. Before the fix this assertion always failed.