Implement iOS CPU and memory sampling in perf payload

[thymikee]

Follow-up to #101.

Goal

Populate real cpu and memory metrics for iOS simulator/device targets in perf payload.

Scope

• iOS simulator + physical device
• Session-scoped perf command response
• Sampling metadata (method, unit, measuredAt)

Proposed approach

• Simulator: process-level sampling via simctl spawn + ps/equivalent
• Physical device: profiling-backed sampling (xctrace or equivalent), with clear runtime expectations

Acceptance criteria

• metrics.cpu.available=true for at least iOS simulator
• metrics.memory.available=true for at least iOS simulator
• Physical-device support documented (implemented or explicit staged limitation)
• Handler returns normalized errors with hints when sampling fails
• Unit tests cover output parsing and response shape
• README + website docs updated for iOS sampling method and caveats

Out of scope

• Android CPU/memory
• FPS sampling