Problem

`UploadPackV2.lazy_fetch?/2` returns true if any want is a non-commit. Consequence: a client that sends 10 commit wants + 1 blob want to a filter-capable server currently gets an unfiltered pack for all 11 objects.

Real promisor lazy-fetches usually send only blob/tree wants, so this is a theoretical corner case — but it’s silent behaviour the README doesn’t call out.

Fix plan

Either:

• A. Partition wants: treat blob/tree wants as lazy (filter bypassed for those SHAs) and commit wants as normal (filter applied to the reachability walk). Two separate walks merged for the final pack.

• B. Document as-is: if you mix commit and non-commit wants with `filter`, the server treats it as a lazy fetch and bypasses the filter. Add to the README capability matrix.

A is correct; B is cheap.

Acceptance criteria

• Integration test: a mixed want-set with one commit + one specific blob + `filter=blob:none` yields a pack containing the commit’s trees (no other blobs) plus the specifically-wanted blob.
• Or: README calls out the behaviour in the filter row of the capability matrix.

Context

Flagged in the audit of PR #20.