Stage 4 of #337. Lanes could only be drawn in the song tab. Now the agent can set them in one step and sees which parameters a lane of a track can move. - New command set_song_lane. The target is named as it is stored: volume, pan or param:<name>. All points come at once, each with tick, value and ease, from the same eases as animation keys, curve assets included. - Points on one tick keep the order they come in, which makes a jump. points [] removes a volume or pan lane, remove a parameter lane. - It refuses with a clear message: a value outside the range, an unknown ease, a missing curve asset, a range for volume or pan, and a new lane for a parameter nothing on the track has. The message then names the parameters there are. - Without min and max a new parameter lane gets the range the dialog in the song tab offers. - get_asset names under params of each song track the parameters a lane can move, with their default. The lanes were already in it. Live in the user's project: get_asset named both parameters of their two lanes (frequency, env) with defaults 20000 and 0.5. Playing to 3 s and 4.2 s, both tracks were heard, the audio clock running. Tests: - agent-music 13/13 (2 new), agent and MCP tests 129/129 - 6 counter-probes, all compile, all red - gate on atx: CHECK EXIT 0, 330 files, 5082 tests Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>