fix(screenshot): capture full Game view — multi-camera + Screen Space-Overlay UI (#33)

[travlake]

Problem

screenshot --view game returns a black / UI-less image on projects whose Game view is composited from multiple stacked cameras and/or whose HUD lives on Screen Space - Overlay canvases (#33).

Root cause

CaptureGameView rendered a single camera.Render() (Camera.main, else an arbitrary FindObjectOfType<Camera>()) into an offscreen RenderTexture. That cannot reproduce a multi-camera stack, and camera.Render() never includes Screen Space - Overlay UI — overlay canvases are composited onto the back buffer after camera render. So the capture came back black / with the UI missing.

Fix

Capture the actual composited back buffer via ScreenCapture.CaptureScreenshot (every stacked camera + all UI, including overlay), awaited end-of-frame using the router's existing async Task<object> handler support (same pattern as RunTests/ManageEditor).

• Completion via PNG IEND chunk — the wait checks for the trailing IEND bytes rather than length > 0, so a partially written file is never returned (ScreenCapture's write is not atomic).
• Bounded wait — by both frame count (120) and wall-clock (3s), and torn down on beforeAssemblyReload, so an in-flight capture can never leak the update subscription or hold the command lock indefinitely.
• Fallback — if no frame renders in time (no Game view, batch mode), falls back to a depth-ordered multi-camera composite that honors each camera's clear flags. Its response message explicitly warns that Screen Space - Overlay UI is omitted (so the degradation is never silent). Note: camera.Render() compositing assumes the built-in render pipeline.

Scene view behaviour is unchanged (single SceneView camera). Adds super_size (1-4) for the primary path; width/height continue to size the fallback.

Verification

Built against the connector in Unity 2021.3.45f2 (built-in pipeline) and exercised end-to-end through the live connector: screenshot --view game now captures the full Game view — multi-camera background and all UI canvases — in both edit mode and play mode, where it previously returned solid black.

Fixes #33.