Expose engine runtime and P2P metrics

[jevgeni17]

Goal

Expose safe, typed measurements of the managed engine's P2P activity and computer-resource use.

Scope

• Add typed SessionStatistics for status, active peers, download/upload rates, and downloaded/uploaded totals from the validated Ace Stream session statistics endpoint.
• Add typed EngineRuntimeMetrics for container state, uptime, CPU usage, memory usage/limit, process count, and cache usage/configured limit.
• Obtain Docker metrics only for the labelled EngineKit-managed container using fixed Process argument arrays; never execute a shell or accept arbitrary container names or Docker arguments.
• Do not label Docker network transmit bytes as P2P upload: they also include the local HTTP stream sent to IINA. Ace Stream session statistics are authoritative for peer transfer figures.
• Provide bounded, cancellable snapshot collection suitable for periodic UI refresh.
• Represent unsupported, temporarily unavailable, and stale measurements explicitly. Metrics failure must never interrupt playback.
• Keep content IDs, stream URLs, VPN keys, and arbitrary Docker output out of logs and diagnostics.

Acceptance criteria

• The API reports session statistics and runtime metrics with clear units and timestamps.
• Collection tolerates engine startup, shutdown, replacement, and temporary API failures.
• Collection cannot target an unrelated container and has bounded frequency and overhead.
• Tests cover HTTP and Docker metric parsing, unavailable fields, stale data, cancellation, and exact process arguments.
• Docker network I/O is never exposed as peer upload/download.

Non-goals

• Setting resource or bandwidth limits.
• Rendering statistics in the macOS application.

Consumer

AceStreamPlayer #3 will present these metrics.