fix(code-demo): impact mode updates per commit when navigating DAG

- Replace hardcoded COMMIT_EDGES[5] with COMMIT_EDGES[state.cur] in selectSymbol so impact highlighting and callee/caller info always reflect the currently selected commit's call graph - Re-apply selectSymbol in selectCommit when graphMode==='impact' so navigating commits in impact mode re-computes the impact subgraph against the new commit's edges