docs: add Swap/Clear/CAS to doc.go ops list + Quick Start

[millerjp]

Summary

doc.go line 27 lists stdlib-parity ops as "Load, Store, LoadOrStore, LoadAndDelete, Delete, and Range" — missing Swap, Clear, CompareAndSwap, CompareAndDelete. Quick Start only shows Store/Load/Range. A reader landing on pkg.go.dev doesn't see the package-level function syntax.

Scope

• Update doc.go ops enumeration to include Swap and Clear.
• Add a CompareAndSwap one-liner to the Quick Start (or cross-reference).

Acceptance criteria

1. doc.go mentions all wrapped sync.Map methods.
2. Quick Start shows package-level CompareAndSwap syntax.
3. llms-full.txt regenerated.

Source: code-reviewer + docs-writer.