Bundles the library-side work for Anvil epic #215 into one PR per repo. Two commits, two REQs, both backed by failure-injection proof. ## What's in here ### T1 — Walker invariant property test (REQ-GIT-076) \`stream_data\` generator produces random commit graphs (1-3 chains of length 1-6, optional merges, optional mode-160000 gitlinks whose SHA targets either the root commit or the current tip). For every generated graph: assert \`MapSet.new(walker_pack_shas) == bfs_reachable(repo, wants)\`. Catches the entire class of walker bugs that hit production in May 2026 (the gitlink-reachability regression — 1043 reachable objects silently pruned from \`fangorn/hephaestus\`'s pack). **Failure-injection proof.** Temporarily reverting the \`mode: \"160000\"\` head in \`collect_single_tree_entry\` makes the property fail on iteration 0: \`\`\` Failed with generated values (after 0 successful runs): Clause: spec <- graph_spec() Generated: %{..., inject_gitlink?: true, gitlink_target: :self, ...} walker missed reachable SHAs: [\"087267660a...\", \"209f7076a8...\", ... 8 SHAs total ...] \`\`\` Restored: 80 random iterations pass in ~100 ms. Adds \`{:stream_data, \"~> 1.1\", only: :test}\` to deps. ### S1 — Stream walker objects through writer (REQ-GIT-080) After \`collect_objects_maybe_shallow\` returns the walker's \`[{type, content, sha}, ...]\` list, project it down to \`[{type, sha}, ...]\` and let the original content list go out of scope (so it's GC'd). Pipe the SHA list through a \`Stream.map(fn {type, sha} -> ... ObjectResolver.read(repo, sha) ... end)\` into a new \`Writer.generate_stream_enum/4\`. Pack-write phase peak heap is bounded by one object at a time. Surface changes: - **New** \`Writer.generate_stream_enum/4\` — accepts \`Enumerable.t/0\` + explicit count. The pack format needs the count in the hashed header, so it cannot be deferred. Same byte output as the existing \`generate_stream/3\`. - \`generate_stream/3\` becomes a thin wrapper that supplies \`length/1\` for list inputs. Same public contract. - \`UploadPackV2.stream_packfile_response\` uses \`drop_content/1\` + \`stream_object_contents/2\` helpers to set up the lazy stream. **Measured impact** (Anvil REQ-GIT-077 budget test, 10 MiB random-blob fetch, avg peak BEAM heap delta over 3 runs): | Library version | Peak heap delta | |---|---| | main (eager) | 14.7 MB | | **this branch** | **3.5 MB** | ~4× reduction on the test fixture. Scaled to prod hephaestus (~210 MB pack), absolute savings should exceed 200 MB. The reduction is in the pack-write phase; the walk phase still allocates content (will be addressed by S4 / pass-through packed object reuse in a follow-up). ## Test plan - [x] \`mix test\` — 945 tests, 0 failures (51 excluded) - [x] \`mix format --check-formatted\` clean - [x] \`mix credo --strict\` clean - [x] Property test demonstrated to fail on the gitlink shape when the fix is reverted - [x] S1 byte output verified equivalent to non-streaming via existing v2 byte-equivalence tests ## Tracks Epic Anvil #215 (REQ-GIT-076, REQ-GIT-080).