Summary

• Fix all 4 existing Dialyzer warnings (type mismatches in diff.ex, pack/reader.ex, protocol/receive_pack.ex)
• Add mix dialyzer step to .anvil.yml CI pipeline
• Any new Dialyzer warning now fails the build

Closes #20

Changes

• diff.ex: diff_blobs spec was missing the {:ok, :binary, sizes} return variant
• pack/reader.ex: Exposed read_object/4 with external resolver param, making the REF_DELTA code path reachable (previously dead code)
• protocol/receive_pack.ex: Added missing pre_receive_hook and cmd_buffer fields to state() type; targeted @dialyzer {:no_opaque, init: 2} for known MapSet opaque type limitation
• .anvil.yml: Added dialyzer CI step depending on compile

Test plan

• mix dialyzer passes with 0 errors locally
• mix compile --warnings-as-errors passes
• mix format --check-formatted passes
• mix test — 529 tests, 0 failures