[Lower priority] Expose performance metrics (FPS, memory, CPU, startup time) #101

Priority: Lower / backlog.

The agent has no way to know if the app it built is fast. Exposing metrics such as FPS, memory usage, CPU, and startup time (e.g. time to first frame or first interactive) would allow agents to optimize for performance. Could be a single metrics or perf command that returns a JSON blob; implementation could use Xcode instruments, Android profiler, or OS APIs where available.

Acceptance criteria (high level):

• Command or mode that returns at least one of: FPS, memory, CPU, startup time
• Document platform support and sampling method