Turbopack: Integrate merge_chunks into production.rs
Refactor production.rs to call merge_grouped_chunks() from merge.rs
instead of having its own copy of the merge algorithm. This eliminates
~500 lines of duplicated merge logic.
production.rs now:
1. Resolves Vc values and groups items by bitmap (unchanged)
2. Converts groups to GroupInput (size + bitmap + batch_group_id)
3. Calls merge_grouped_chunks() for the merge + absorption pass
4. Maps results back to make_chunk() calls
merge.rs now exposes two entry points:
- merge_grouped_chunks(): takes pre-grouped items (used by production.rs)
- merge_chunks(): groups items by bitmap first, then calls
merge_grouped_chunks() (used by unit tests)
Test-only types (ChunkItemForMerging, MergedChunkInfo, merge_chunks,
hash_bitmap) are gated with #[cfg(test)].
