Implement SharedArrayBuffer + Atomics #4794

Why

~391 Atomics + ~524 SharedArrayBuffer tests. Most agent-based ones use the excluded $262.agent harness, but basic SAB construction and non-agent Atomics are judged and currently fail. Node supports both. CLAUDE.md currently lists "No SharedArrayBuffer or Atomics" as a non-goal — this issue flips it to a goal.

Scope

• SharedArrayBuffer with a real shared-memory backing.
• Atomics: add/and/or/sub/xor/load/store/exchange/compareExchange/wait/notify/isLockFree.
• Integrate with the existing perry/thread OS-thread model (introduces shared mutable state across threads).

Acceptance

Non-agent built-ins/Atomics + built-ins/SharedArrayBuffer tests pass. Agent-based tests tracked separately (need a $262.agent harness hook).

Effort

Large (threading + memory model).