index_diagnostics_metrics

For each sync / index run on a base, record:
- wall-clock * * sum / total duration * * of the pass
- * * file count * * ( visited ), skipped-by-mtime count, indexed ( parse / persist attempted ) count, error count
- * * average path depth * * of visited files ( segments under the base root )
- per-file * * duration * * and outcome ( `mtime_skip` | `mtime_refresh` | `hash_skip` | `persisted` | `error` )
Runs are attributable to a trigger ( `soft_sync` | `rebuild` | `enqueue` | `stale` ).
Instrumentation wraps workspace-index.ts soft sync and nonblocking_index single-file paths via index-diagnostics-store.ts.

Status: done

Interactive page: index_diagnostics_metrics