Describe Preset, Randomize and Mutate in Instruments (#329)

Development counts 1847 checks.

Development.md

@@ -18,7 +18,7 @@ php tests/php/accounts.test.php                               # the API's accoun
                                                               #   has and the web server's container has not (SKIP there)
 php tests/php/snapshots.test.php                              # the snapshot store (#231), the same way
 php tests/php/releases.test.php                               # the release store (#266), the same way
-node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1843 editor checks, screenshots in /tmp/gm-e2e-*.png
+node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1847 editor checks, screenshots in /tmp/gm-e2e-*.png
 GM_STEPS=20 node tools/e2e.mjs                                # or 1-12, or 6,20 — a subset while working on one step
 ./e2e.sh start [GM_STEPS]                                     # the same run detached, see below
 ./e2e.sh status                                               # live progress, or the last run's result

Instruments.md

@@ -218,6 +218,35 @@ so a drum can be hit quickly. **Stop**, or `Esc` on the keys, ends everything th
 instrument has a number field in the header, starting at its default: what the keys play with,
 heard at once on what is playing. Those values are the tab's own and never written into the asset.
 
+### Presets, Randomize and Mutate
+
+Three buttons in the header make 8-bit sounds the way sfxr does (#329). Each is one undo step and
+plays the note last played at once (C4 before any), for up to 2 s.
+
+- **Preset** opens a menu: *Pickup*, *Laser*, *Explosion*, *Powerup*, *Hit*, *Jump*, *Blip*. It
+  puts a new graph in place of the instrument's: Notes for all 128 notes → [Chip](#chips) →
+  Envelope → Filter → Bitcrusher → Output. Groups and notes on the canvas stay. Every click draws
+  the sound anew within its kind, with sfxr's generator for it. The Chip plays at its own
+  frequency, *Follow note* off, so every key plays the same sound, as an effect does; turn it on to
+  play the sound as a melody. The Envelope has *Sustain* 0, so the sound ends by itself.
+- **Randomize** draws every number of the Chips, Envelopes, Filters and Bitcrushers in the graph
+  anew, as sfxr's randomizer does: most of them wild. It leaves the wave, *Follow note*, the type
+  of a Filter, the *Mix* of a Bitcrusher, an Envelope's *From* and *To* and every other node as
+  they are. An Envelope gets *Sustain* 0. An input with an edge into it keeps what it is fed.
+- **Mutate** moves the numbers of those nodes a little, each one in two: a frequency, cutoff or
+  rate by up to a quarter of an octave, the rest by a step and a tenth of its value. A 0 stays 0,
+  so nothing that was off comes on, and *Sustain* stays as it is.
+- Randomize and Mutate are grey while the graph has none of those nodes.
+- **Audition** (off when the tab opens, as in the Pattern tab): on, every change to the graph
+  plays the note last played again, 150 ms after it rests, so dragging a number is heard once you
+  let it be. Moving a node is no change.
+
+sfxr counts its sliders from 0 to 1; the presets turn them into Hz, semitones and seconds with
+sfxr's own formulas. Its envelope holds a sound and lets it fall in a straight line, an Envelope
+falls away quick at first, so the decay is three times sfxr's hold and fall. sfxr's punch and its
+filter sweeps are left out, its phaser is #341. A Preset with both of sfxr's filters keeps the
+low-pass.
+
 ## Playing one in a game
 
 **Play note** plays one note of an instrument:
@@ -281,6 +310,7 @@ the game's, so a slow frame or a busy page never moves a note. The Pattern tab p
 
 ## Not yet
 
-No oscillators or 8-bit sources (#329), no swing. An Envelope
+No swing. No view of the waveform a Chip makes, no import of sfxr or jsfxr codes, no WAV export
+(#340), no phaser (#341). An Envelope
 shapes notes of an instrument only, not a sound played through a sound FX. Notes can not be played from an `.mid` file or a MIDI
 keyboard.