Implement FPS sampling for perf metrics

[thymikee]

Follow-up to #101.

Goal

Expose real FPS / frame pacing metrics in perf payload instead of placeholders.

Scope

• Add platform-specific FPS sampling strategy and response fields
• Start with Android (dumpsys gfxinfo ... framestats), then define iOS path (xctrace/CoreAnimation or equivalent)

Acceptance criteria

• At least one platform returns metrics.fps.available=true with numeric values
• FPS sampling method and caveats are included in payload metadata
• Unsupported targets still return explicit unavailable reason
• Unit tests cover parser/summary logic and handler integration
• Docs explain what FPS means here and how it differs from recording FPS

Notes

• Keep existing startup metric behavior unchanged
• Ensure parsing is resilient to OS/version output format drift