Adds Maestro App Percy Integration

Cross-platform Maestro view-hierarchy resolver (packages/core/src/maestro-hierarchy.js):
- Android: gRPC primary → maestro-CLI shell-out → adb-uiautomator dump cascade
- iOS: HTTP /viewHierarchy POST primary → maestro-CLI shell-out cascade
- Three-class gRPC error taxonomy (schema / channel-broken / contention) with class-specific routing

• Screenshot relay (/percy/maestro-screenshot in api.js): accepts new filePath field from SDK (eliminates the legacy

SCREENSHOTS_DIR glob coupling) +
ignoreRegions/considerRegions algorithm pass-through + PNG-header-derived tag.width/height (no more host injection)

Observability (/percy/healthcheck): maestroHierarchyDrift envelope with per-platform {lastFailureClass, fallbackCount, succeededVia, code, reason, firstSeenAt} —
two-slot, set-once schema-class drift bit