Inspect: make layer list opt-in

[yuiseki]

Summary

• add --include-layer-list flag and disable layer stats by default
• show a tip when layers are omitted in text reports
• document the new option in docs/SPEC.md

Testing

• make fmt
• make clippy
• make test

Checklist

• I updated documentation where needed
• I added or updated tests if behavior changed
• I considered privacy/security implications