perf: document sccache setup for macOS developers

[bug-ops]

Problem

Project does not document sccache configuration, which provides 10×+ build speedup for incremental builds on macOS.

Impact

• Developer productivity: 30-60 second incremental builds → 3-6 seconds
• CI potential: Could speed up CI builds significantly

Solution

Add to CONTRIBUTING.md or docs/src/development.md:

## Build Optimization (macOS)

### Install sccache
\`\`\`bash
brew install sccache
# or
cargo install sccache --locked
\`\`\`

### Configure
\`\`\`toml
# ~/.cargo/config.toml
[build]
rustc-wrapper = "sccache"
\`\`\`

### Verify
\`\`\`bash
sccache --show-stats
\`\`\`

Also mention XProtect exclusion (System Settings → Privacy & Security → Developer Tools → Add Terminal.app) for 3-4× speedup.

Priority: P1
Effort: Small (30 minutes)
Related to #391