feat: correlate download diagnostics with node GETs

[dirvine]

Summary

• add schema-v4 request_id and local_peer_id fields to download diagnostics
• capture one correlation object per diagnostic peer GET and use its request ID both on the wire and in the emitted peer-attempt record
• preserve the diagnostics-disabled request path unchanged
• document exact client/node correlation semantics for V2-903

This allows an exact join from the client record to node-side get_rpc telemetry using:

• client request_id = node tail.request_id
• client local_peer_id = node tail.source_peer
• matching chunk_address and request time window

No protocol change is required: the existing chunk GET already carries the request ID.

Base/rollout: this diagnostic runner is intentionally based on rc-2026.7.4 (ant-cli 0.3.2 / ant-core 0.5.1), matching the production investigation build. It is deployed separately and does not require an ant-node or production-client release. Current main has since moved to a newer ant-protocol/saorsa-core dependency line; forward-porting the broader diagnostics branch should be a separate change if the tooling is retained.

Verification

• cargo test -p ant-core --lib — 443 passed
• cargo check -p ant-core
• cargo check --workspace --all-features
• cargo clippy --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check
• git diff --check
• cargo zigbuild --release -p ant-cli --target x86_64-unknown-linux-gnu

Two independent final-diff reviews returned APPROVE, with no critical or important findings.

Behaviour notes

• Diagnostics disabled: no new request allocation, formatting, or I/O.
• Diagnostics enabled: the request ID is allocated once per peer attempt before any await and is retained for success, remote error, transport error, and timeout records.
• Chunk-level records keep request_id and local_peer_id as null, because they do not represent an individual peer request.

Linear: V2-903