feat: add Swap and Clear methods (#13)

[millerjp]

Summary

Wraps sync.Map.Swap (Go 1.20) and sync.Map.Clear (Go 1.23) as methods on SyncMap. Zero API surface surprise — Swap mirrors the LoadAndDelete !loaded guard for typed-zero-on-miss; Clear is a bare pass-through.

Closes #13.

Added methods

func (m *SyncMap[K, V]) Swap(key K, value V) (previous V, loaded bool)
func (m *SyncMap[K, V]) Clear()

Verification

$ make check
… (14/14 gates green, coverage 100%, govulncheck clean)

$ go test -race -count=10 -run 'TestSwap|TestClear|TestSwapContention' ./...
ok github.com/axonops/syncmap 1.300s

$ go test -bench='Swap|Clear' -benchmem -run=^$ -benchtime=50000x ./...
BenchmarkSwap-8              92.9 ns/op   71 B/op   2 allocs/op
BenchmarkSwapAbsent-8       159.9 ns/op  119 B/op   3 allocs/op
BenchmarkSwapParallel-8     226.9 ns/op   71 B/op   2 allocs/op
BenchmarkClear-8             75.2 ns/op  208 B/op   2 allocs/op  (includes one Store per iter)
BenchmarkClearParallel-8     39.4 ns/op  208 B/op   2 allocs/op

Agent gates

• Plan — single-commit scope, placement between Delete and CompareAndSwap.
• code-reviewer (post) — ship it; NITs non-actionable.
• security-reviewer (post) — no blocking; Swap's !loaded guard prevents nil-interface type-assertion panic; atomics + WaitGroup correct.
• performance-reviewer (post) — flagged BLOCKING missing SwapParallel / ClearParallel; both added in a follow-up edit. Now passing.
• go-quality — ALL PASS (14/14).
• commit-message-reviewer — PASS.

Deferrals

• CHANGELOG — docs: add README, CHANGELOG, NOTICE, SECURITY, and LLM docs bundle #17.
• BDD scenarios for Swap/Clear — test: add BDD (godog) suite and fuzz targets #16.
• Full public-method benchmark matrix + raw-sync.Map overhead + bench.txt baseline — test: add benchmarks, runnable examples, and bench-regression infrastructure #15.

Test plan

• CI green
• Coverage remains 100%
• Contention test stable under -race × 10