CI: cache Rust builds in release workflow

[ElbertePlinio]

release.yml has no Rust build caching, so every tagged release compiles the full workspace from scratch on all platforms (v0.1.2 release run: 22 min). ci.yml already uses Swatinem/rust-cache@v2; pickscribe's release workflow has it and releases in 5–10 min.

Checklist

• Add Swatinem/rust-cache@v2 to release.yml build matrix (after rust-toolchain step), mirroring pickscribe
• CI green

Validation

• Next tagged release: faster builds on the 2nd release after merge (warm cache), assets + latest.json intact

[ElbertePlinio]

Part of the agent-DX batch: pickforge/pickforge-platform#19

[ElbertePlinio]

Merged in #32 (rust-cache scoped to src-tauri per review). Proves out over the next two tagged releases.