Editor S12: Embedded Play mode

[mblackman]

P2.

Run the game inside an editor viewport instead of spawning a separate OS window (the #48 Play button shells out to a standalone process).

Scope:

• Render the player into an editor dockspace viewport (engine renders to an offscreen render target the editor blits/samples)
• Input routing/focus into the embedded view; pause/stop/restart controls
• Surfaces in the Devices panel (Editor S8: Devices panel #77) as the "Local embedded" row

Standalone Play (#48) stays as the out-of-process option; this is the in-editor complement.

Depends on #48 (play button). Parent: #45.