Function detect_dead_code 

pub fn detect_dead_code(
    symbols: &[SymbolNode],
    edges: &[Edge],
    config: &DeadCodeConfig,
) -> DeadCodeAnalysis

Detect dead code: symbols with zero incoming usage-edges, after applying exclusion layers (entry points, exports, tests, migrations, user patterns).

Complexity: O(E + S) — single pass over edges, single pass over symbols.