build: add sccache for faster local and CI builds

- Add .cargo/config.toml with rustc-wrapper = sccache for local caching
- Install sccache in CI clippy/test jobs via prebuilt musl binary
- Add actions/cache to persist sccache dir across CI runs (keyed by Cargo.lock)
- Add BuildKit cache mounts in Dockerfile for sccache across Docker builds
- Disable sccache for fmt/audit jobs where it adds no value

Local clean-rebuild speedup: 1m58s -> 34s (3.4x) with warm cache.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.cargo/config.toml

@@ -0,0 +1,5 @@
+[build]
+# Use sccache as the rustc wrapper for compile caching.
+# Falls back gracefully: if sccache is not installed, cargo will warn but
+# still compile. Install with: cargo install sccache
+rustc-wrapper = "sccache"