Add CLAUDE.md with architecture overview

[styler]

Summary

• Adds CLAUDE.md documenting how this plugin actually fits together, so future Claude Code sessions (and humans) can get oriented without re-reading 130+ files.
• Covers: build/format/CI, the StreamElementsGlobalStateManager subsystem graph, the composition layer (lazy video encoders via SELazyOBSVideoEncoderProvider, private OBS canvases via obs_canvas_create_private/obs_canvas_scene_create), the multistreaming output engine (parallel obs_output_t instances bound to shared encoders), the JSON-over-localhost-websocket host API (204 flat handlers wired via API_HANDLER_BEGIN macros, threading via QtPostTask onto the Qt main loop), and the scene/profile/backup layer (including the Windows-only AngelScript restore-script-host).
• Concrete identifiers (class names, OBS API symbols, port 27952, pipe \\.\pipe\StreamElementsObsLiveControllerServer, config file obs-streamelements-core.ini, handler-name shape) were grep-verified before writing.

Test plan

• Skim CLAUDE.md and flag anything that's wrong, outdated, or misleading
• Confirm the description of the obs-browser sibling-repo build requirement matches how engineers actually set up local builds
• Confirm the multistreaming description matches the intended model (especially the lazy-encoder sharing claim)