Function incremental_index 

pub fn incremental_index(
    root: &Path,
    backends: &[&dyn EmbedBackend],
    tokenizer: &Tokenizer,
    cfg: &SearchConfig,
    profiler: &Profiler,
    model_repo: &str,
    cache_dir_override: Option<&Path>,
    repo_level: bool,
) -> Result<(HybridIndex, ReindexStats)>

Load or incrementally update a persistent index.

1. Resolve cache directory
2. If manifest exists and model matches: Merkle diff, re-embed dirty files
3. If no manifest: full embed from scratch
4. Rebuild SearchIndex from all cached objects

Errors

Returns an error if embedding fails or the cache directory is inaccessible.