ci: enable cargo caching for clippy workflow

[huth-stacks]

What

Enable the built-in cargo/target caching in the clippy workflow.

Why

The clippy workflow (clippy.yml) explicitly sets cache: false in the actions-rust-lang/setup-rust-toolchain step. This forces a full recompilation of the entire workspace on every PR, even when only a few files changed. Enabling caching allows incremental compilation for clippy analysis.

The Change

In .github/workflows/clippy.yml:

-          cache: false
+          cache: true

1 line changed in 1 file.

How to Verify

• First run: similar timing to baseline (cold cache)
• Second run on same branch: should show significant speedup (~50-75% faster)

Metrics to Track

Metric	Baseline	Expected (warm cache)
clippy duration	4-8m	~1-2m

Security Checklist

• No new permissions granted
• No secrets exposure
• Cache is scoped by repo + branch (GitHub Actions default behavior)

Part of CI Optimization Series

PR 7 of 7. Expected impact: ~3-6 min off clippy (on warm cache).