Add stop action and switch/add playback modes to scene-controller

[mutec]

Summary

Extends the scene-controller with a fourth action and two new node settings.

New action: stop

• Halts output for a scene but keeps it stored (unlike reset, which deletes).
• When the optional Send zero values on stop setting is enabled and the last active scene is stopped, a nulled universe is emitted once so the stage goes dark.

New setting: Playback mode

• Switch (default): a new play replaces the running look.
• Add: scenes are stacked and combined per channel by HTP (highest value wins). stop/reset of one scene re-emits the remaining combined look; the output only goes dark once the last scene ends.

The controller emits DMX values once per change; the downstream sACN-out node keeps streaming them.

Behaviour notes

• reset is unchanged: it deletes the scene and blacks it out if currently playing.
• Outgoing stop blackout messages carry stopped: true (analogous to reset: true).

Verification

• npm run build (tsc + build.js), eslint and prettier all clean; dist/ rebuilt.
• 17 behavioural checks against the compiled node pass: HTP merge, add vs switch, stop with/without blackout, data retention on stop, reset delete+blackout, and no-op on stopping an inactive scene.