Docs: plan unified device model and controller_cli improvements

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

planning.md

@@ -118,4 +118,5 @@ These are open questions tracked in `architecture.md` that do not need to be res
 - Node discovery mechanism
 - Hard vs soft byte budget limits
 - Cooperative capture release: if a capture source has no live downstream targets for a configurable time window, stop capture and release the device. Intended as a resource-conservation policy rather than an immediate reaction to disconnect events. Requires the node to track downstream liveness (e.g. last successful send timestamp per output) and implement a reaper timer.
-- Xorg viewer remote controls: expose viewer state (zoom, pan, scale policy, anchor) as enumerable/settable controls via the protocol, analogous to V4L2 controls. Future extension: shader-based post-processing — initial candidates are a colour-correction shader and custom user-provided GLSL fragment shaders sent over the wire.
+- Unified device model: active display windows should be registered as devices alongside V4L2 cameras, using the same ENUM_DEVICES / ENUM_CONTROLS / GET_CONTROL / SET_CONTROL protocol. START_DISPLAY would return a device_id for the opened window; controls (scale, anchor, position, size, zoom, pan) are then addressable as (device_id, control_id) pairs like any other device. Requires a device_type field in ENUM_DEVICES responses so controllers can distinguish V4L2 devices from display windows. Future device types: codec processes, screen grab sources. This extends naturally to shader-based post-processing and other viewer state as controls.
+- controller_cli improvements: (1) readline for line editing and history; (2) discovery integration — run a discovery listener, expose a `peers` command listing discovered nodes, `connect <id|host:port>` to switch target, auto-print newly discovered nodes.