docs(research): record POC-3 outcome — phase 0 gates all pass

- pocs.md: POC-3 marked complete/proceed with result summary; sequencing
  status updated
- alk-stack.md: verify item 2 (HTTP smart protocol shape) resolved,
  pointing at poc3-findings.md

docs/research/alk-stack.md

@@ -87,6 +87,13 @@ door produced the stream.
 2. **HTTP smart protocol shape**: confirm alkhttp's streaming response body
    can carry a pack (chunked) and that request bodies stream in without
    buffering the whole POST in memory (receive-pack can be gigabytes).
+   **Verified (POC-3, 2026-09-21)**: real git 2.43 clones/fetches
+   fsck-clean over `Connection → HttpAdapter → axum custom routes`;
+   request bodies stream (`BodyDataStream`, one hyper-read chunk per
+   `poll_next` — measured with a dribble probe and git's forced-chunked
+   POST path); pack responses stream under back pressure (bounded mpsc →
+   `Body::from_stream`, O(counts) RSS on a 60k-object clone). See
+   `poc3-findings.md`.
 3. **alkcall backpressure/limits vs git sessions**: large clones are
    long-lived, high-throughput, single-direction streams; confirm channels
    buffer limits (ADR-040) don't fight pack streaming, or route git sessions