Add CI clippy gate for disk-index feature

[Copilot]

cargo clippy --workspace --features disk-index was not exercised in CI, allowing feature-gated lint errors to silently land on main.

CI

• Add clippy-disk-index job running cargo clippy --locked --workspace --all-targets --no-deps --features disk-index on both ubuntu-latest and ubuntu-24.04-arm
• Gate basics on clippy-disk-index so downstream test jobs are blocked on failure

Code fix

• Remove redundant as f64 cast on mean_ios in search_disk_index.rs — only reachable under disk-index, which is why it was never caught before

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[BUG] No gates for disk-index feature</issue_title>
<issue_description>## Expected Behavior
On main branch, code should pass clippy for all feature flags

Actual Behavior

cargo clippy --workspace --features disk-index did not work as of 5ab92c. It seems CI does not catch this.

Error

See PR #834 for error fix.

Your Environment

On all OS, as of commit 5ab92cacce7c2f7aa0832006fa65be7ebdcad38c

</issue_description>

<agent_instructions>Ensure clippy is being run against all feature flags in CI</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes [BUG] No gates for disk-index feature #835

---

🔒 GitHub Advanced Security automatically protects Copilot coding agent pull requests. You can protect all pull requests by enabling Advanced Security for your repositories. Learn more about Advanced Security.

[harsha-simhadri]

subsumed by #838