fix(kernel): core hardening, in-memory coordinator, and resilience fixes

[nexssp]

Summary of Changes

• Idempotency:
  • Implemented IdempotencyCoordinator on MemoryIdempotencyStore with sequence-based phantom delete protection and idempotent Release.
  • Split interface contracts and in-memory coordinator implementation into idempotency.go and idempotency_default.go.
• Observability:
  • Wired SlowLogMiddleware into Builder.LogSlowWhen.
• Performance & Zero-Alloc Guarantees:
  • Optimized Do() execution to avoid allocating execState and context.WithValue unless OnExecuted hooks are present, maintaining 0 allocations on hot paths.
  • Added dedicated regression unit tests in zero_alloc_test.go.
• Resilience & Locking:
  • Guarded against false lease-loss reporting during normal context cancellation/shutdown in ExclusiveFenced.
• Composition & Caching:
  • Pre-built chained actions in Chain() at composition time.
  • Suppressed OnExecuted hooks on cache hits and validated singleflight context isolation.

Verification

• go test ./... — PASS
• go test -race ./... — PASS
• go vet ./... — PASS
• go test -run none -bench . -benchmem ./action/... — 0 allocs/op verified