Adds a docs overlay that lives inside the rack chrome, opened from a new
MANUAL hw-btn in the top bar (ESC also closes). Styled as a screen recess
matching the editor — amber section headers, engraved labels, recessed code
blocks — so it reads as a manual page of the instrument, not a help popup.
Covers patch language, execution model, every node type (osc, trig, adsr,
filter, noise, seq, delay, poly+voice) and the four control nodes (knob,
fader, step_seq, piano_roll), plus practical tips on gain staging and
hot-reload behavior. Each example block carries a TRY button that swaps the
editor contents with the snippet so you can hear it immediately (Ctrl/Cmd-Z
to bring your patch back).
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