feat: surface chunk response transport metadata

[dirvine]

Linear issue

Closes V2-903

Risk tier

• T0 — docs / tooling / CI / pure UX-output. Repo CI only.
• T1 — client-only, no network-facing behavior change. CI + prod compat smoke.
• T2 — node/client logic with behavioral surface, no protocol/format/economics change. Dev testnet + ADR.
• T3 — protocol / storage format / payments / routing. T2 evidence + adversarial testing.

Compatibility

• Wire: none; existing messages and response filtering are unchanged.
• Storage: none.
• API: additive ChunkProtocolResponse envelope and send_and_await_chunk_response_with_metadata; the existing helper remains as a compatibility wrapper.

Semver impact

• breaking
• feature
• fix

Test evidence

• cargo fmt --all -- --check — passed.
• cargo clippy --all-targets --all-features -- -D warnings — passed.
• cargo test --all-features -- --test-threads=1 — passed: 79 tests.
• cargo build --no-default-features — passed.

Current stacked-PR status: the publish dry-run is expected to remain red until saorsa-core #162 is released and this PR switches from the temporary git override to that released version. All other current checks pass.

New dependency

none. This PR temporarily follows the branch from WithAutonomi/saorsa-core#162 so the stack is testable; replace it with that crate's released version before merge.

ADR

n/a — the change exposes metadata already present on received events and does not alter the wire protocol or response acceptance.

Mitigation / rollback

Revert this commit; existing callers continue through the unchanged compatibility helper.