chore(ci): test-gate release builds with clippy + cargo test

[orrinfrazier]

Closes #17

Summary

• release.yml built release binaries with no test/clippy step — the exact release commit+toolchain was never test-gated (only main CI + the manual misc/RELEASE_CHECKLIST.md covered it).
• Adds Clippy + Test steps to the build job before Build, mirroring ci.yml, so a lint/test failure fails the job before any binary is produced or uploaded.
• Gating runs per-OS across the existing matrix, so each released binary is validated on its own OS + toolchain.

Changes

Single file: .github/workflows/release.yml

• env: add RUST_BACKTRACE, RUST_MIN_STACK, CMD_CLIPPY, CMD_TEST (copied verbatim from ci.yml).
• Install Rust step: add components: clippy.
• New steps before Build, in order: Install dependencies (Windows) (lukka/get-cmake, for randomx-rs) → Download monerod (./.github/actions/monerod-download, required by the test suite) → Clippy (fail on warnings) → Test.
• Final step order: Checkout → Install Rust → Install deps (Windows) → Download monerod → Clippy → Test → Build → Generate Archives → Archive.

Why "add steps" over "require a prior CI run"

Release is workflow_dispatch on an arbitrary inputs.commit that may never have had a CI run, and since stable floats, a prior run wouldn't validate the same toolchain. Gating in-job is the only approach that tests the exact release commit+toolchain — the issue's core complaint.

Deliberately out of scope

• No toolchain pinning (tracked separately as S06-1).
• No build caching (keeps release builds clean/reproducible).
• Pre-existing ShellCheck warnings in the unchanged Generate Archives block (not introduced here; this repo's CI does not run actionlint).

Testing

• actionlint .github/workflows/release.yml: no new findings vs origin/main (the 6 ShellCheck warnings are pre-existing in the untouched Generate Archives block).
• 13/13 structural acceptance assertions pass (env vars, clippy component, monerod/cmake steps, step wiring/order, ref preserved, toolchain unpinned, Build preserved).
• Reviewed via cross-family consensus (Claude + Gemini + Codex). No blockers.

Resolves issues/S06-2-release-test-gate.md