Function upsert_reference_cache_v2_with_report 

pub fn upsert_reference_cache_v2_with_report(
    config: &FoundryConfig,
    build: &CachedBuild,
) -> Result<CacheSaveReport, String>

Incrementally upsert v2 cache shards from a partial build (typically a saved file compile). This is a fast-path: it updates per-file shards and file hashes for touched files, while preserving existing global metadata.

The authoritative full-project cache is still produced by full reconcile.