perf(chunk): share chunk Bytes across close-group fan-out (drop to_vec)

[jacderida]

Summary

• Drop content.to_vec() at ant-core/src/data/client/chunk.rs:173 and pass the refcounted Bytes straight through to ChunkPutRequest::with_payment.
• Pin ant-protocol to perf(chunk): hold ChunkPutRequest content as Bytes for zero-copy fan-out ant-protocol#6 via [patch.crates-io] so the new content: Bytes field type resolves until that PR lands and is released to crates.io.

Why this matters

A heaptrack capture of a release ant binary uploading a 20 MB file on a 4 GB client VM (DEV-01 testnet) shows peak heap ≈ 285 MB pinned to alloc::raw_vec::RawVecInner::finish_grow, with 168 MB consumed via ChunkMessage::encode → chunk_put_to_close_group. Trace summary:

peak heap memory consumption: 570.98 MB    (for a 20 MB file)
total memory leaked: 22 MB                  ← clean exit, no real leak
total runtime: 53 s

Source of the bloat: chunk_put_to_close_group spawns CLOSE_GROUP_MAJORITY ≈ 5 per-peer tasks via FuturesUnordered, each calling chunk_put_with_proof, which until now did:

let request = ChunkPutRequest::with_payment(address, content.to_vec(), proof);

Even though content arrives as a refcounted bytes::Bytes, that .to_vec() deep-copies the whole 4 MB chunk into a fresh Vec<u8> — once per recipient. Combined with the AIMD store concurrency cap at 64 (raised from the prior static 8 in commit 768e3bd in late April), peak in-flight chunk content reaches:

64 chunks × 5 peers × 4 MB-copy × 2 (copy + encode buffer) ≈ 2.5 GB

which is exactly the gap between the 256 MB peak the spilling design intended (file.rs:130 comment) and the actual ~3.55 GB anon-rss observed at every OOM kill on the DEV-01 1 GB / 500 MB / 300 MB uploaders. Lowering the cap would also restore 4 GB-VM viability, but this PR addresses the root cause: the per-recipient copy.

With this change, each peer's spawned task holds the same single 4 MB backing buffer via Bytes' refcount — N copies → 1 copy per chunk.

Wire compatibility

Unchanged. Bytes and Vec<u8> serialise identically under postcard/serde (varint length + raw bytes). Mixed networks of patched/unpatched clients and servers interoperate.

Patch.crates-io stanza

This PR adds:

[patch.crates-io]
ant-protocol = { git = \"https://github.com/jacderida/ant-protocol.git\", rev = \"4aac7d3...\" }

The pin is against the perf branch on the jacderida fork; the ant-protocol PR is at WithAutonomi/ant-protocol#6. The stanza should be removed once that PR lands and a crates.io release containing it is published.

Test plan

• cargo check --workspace — clean
• cargo check --bin ant — clean (the deployable binary builds)
• heaptrack-derived reasoning verified against captured .zst profiles
• Deploy: cherry-pick onto the DEV-01 testnet branch, build via saorsa-deploy build ant, re-provision a 1 GB / 500 MB / 300 MB uploader on a 4 GB VM, confirm OOMs cease and memory stays well below the 3.55 GB ceiling

Known follow-up

cargo test --workspace currently fails on a transitive dev-dep build of ant-node 0.11.1, which reads request.content as Vec<u8> in storage/handler.rs:239. The deployable ant binary doesn't link ant-node, so this doesn't block the operational fix, but ant-node will need a matching update when its protocol pin advances — out of scope for this PR.

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