chore(storage): add joinable chunk GET telemetry

[dirvine]

Linear issue

https://linear.app/autonominetwork/issue/V2-995/add-joinable-chunk-get-timing-telemetry-for-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.

This is an output-only observability change. It does not alter node decisions, message encoding, stored data, routing, or network bytes. A human reviewer should confirm the proposed tier.

Compatibility

• Wire: none; the existing ChunkMessage.request_id and response encoding are unchanged.
• Storage: none; the LMDB read path and stored-data format are unchanged.
• API: no public API change; receipt context and pending telemetry remain crate-private.

Semver impact

• breaking
• feature
• fix

Test evidence

• cargo fmt --all -- --check — passed.
• cargo clippy --all-targets --all-features -- -D warnings — passed.
• cargo test storage::handler --no-fail-fast — passed: 19 tests.
• Local success and not-found GET tests preserve the existing request ID and chunk address while asserting the authenticated source, queue wait, and bounded storage outcome used for exact client/node joins.
• cargo test --all-features --no-fail-fast — all 925 library tests passed. Two unrelated targets failed: one E2E node transport setup failure passed its exact rerun; enforced_floor_rejects_cheapest_of_k_real_settlement also fails at unchanged base d688747 with the same assertion.
• Exact diff independently reviewed for correctness/lifecycle and observability. GLM-5.2 returned PASS with only shutdown-only/style notes. A second local review raised send-failure and semaphore concerns, but those were rejected against the code: send failures map to send_error, and the 64-permit semaphore/task arrangement is pre-existing.

New dependency

None.

ADR

n/a — proposed T0 output-only observability change; no architecture, protocol, storage, or behavioural decision.

Mitigation / rollback

Revert this PR; the telemetry is process-local and has no persisted or wire state to unwind.