MIDI Documentation and optional mouse event support

docs/midi_input.md

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+# MIDI Input
+
+Read incoming MIDI control change values with the `ccval` word. This lets you use hardware controllers to modulate parameters in your scripts.
+
+## Reading CC Values
+
+The `ccval` word takes a CC number and channel from the stack, and returns the last received value:
+
+```forth
+1 1 ccval   ;; read CC 1 (mod wheel) on channel 1
+```
+
+Stack effect: `(cc chan -- val)`
+
+The returned value is `0`-`127`. If no message has been received for that CC/channel combination, the value is `0`.
+
+## Device Selection
+
+Use `dev` to select which input device slot to read from:
+
+```forth
+1 dev 1 1 ccval   ;; read from device slot 1
+```
+
+Device defaults to `0` if not specified.
+
+## Practical Examples
+
+Map a controller knob to filter cutoff:
+
+```forth
+74 1 ccval 127 / 200 2740 range lpf
+```
+
+Use mod wheel for vibrato depth:
+
+```forth
+1 1 ccval 127 / 0 0.5 range vibdepth
+```
+
+Crossfade between two sounds:
+
+```forth
+1 1 ccval 127 /    ;; normalize to 0.0-1.0
+dup saw s swap gain .
+1 swap - tri s swap gain .
+```
+
+## Scaling Values
+
+CC values are integers `0`-`127`. Normalize to `0.0`-`1.0` first, then use `range` to scale:
+
+```forth
+;; normalize to 0.0-1.0
+74 1 ccval 127 /
+
+;; scale to custom range (e.g., 200-4000)
+74 1 ccval 127 / 200 4000 range
+
+;; bipolar range (-1.0 to 1.0)
+74 1 ccval 127 / -1 1 range
+```
+
+The `range` word takes a normalized value (`0.0`-`1.0`) and scales it to your target range: `(val min max -- scaled)`.
+
+## Latency
+
+CC values are sampled at the start of each step. Changes during a step take effect on the next step. For smoothest results, turn knobs slowly or use higher step rates.