Problem

`UploadPackV2.lazy_fetch?/2` calls `ObjectResolver.read/2` on every `want` to detect promisor lazy-fetch requests (where the want points to a blob or tree, not a commit). For a clone with many wants (34 wants on a typical anvil clone), that’s 34 extra storage round-trips before the walk even starts.

Fix plan

Add `ObjectResolver.read_type/2` (or `get_object_type/2`) that reads only the object-header bytes (` \0…`) without decompressing the full content. Use that in `lazy_fetch?/2`.

Cheap variant: for loose objects, read the first ~64 bytes from storage and decompress only that. For pack-stored objects, read the pack entry header (type + size varint) without the body.

Acceptance criteria

• `lazy_fetch?/2` performs zero full-object reads in the common (all-commits) case.
• A before/after microbenchmark shows a ~50% reduction in fetch-setup time on a clone with 30+ wants.

Context

Flagged in the audit of PR #20; tracked to follow-up.