Show a Saturation node as a device that glows with its drive (#396)

A Saturation in the graph was a stack of rows. The user wanted a device
of its own style and size, like the Delay and the Reverb, that shows
warmth and heat the harder the sound is driven.

- The node is a copper plate of 720x280 with a button per type, the
  auto-gain lever, a big drive knob and small knobs for tone, output
  and wet. Each change is one undo step, wired numbers lock their knob.
- The screen draws a sine at -12 dBFS, the level auto-gain is reckoned
  at, as a grey ghost. Over it is the wave that comes out: bent by the
  type, its DC taken out, auto-gain and output after it, blended by
  wet. Under it stand the auto-gain and the peak.
- The heat is the drive over 48 dB, as far as wet lets it be heard.
  The wave goes from cold steel through ember red to white, a glow
  rises from below and flickers, and embers fly up from about 8 dB on.
  A lamp beside the name glows with it.
- The node keeps the editor drawing only while there is heat. Without
  drive it draws nothing, unlike the Delay, which always animates.
- The Reverb's private type button is now PickButton with its own
  colours, and the Saturation's type buttons use it as well.
- No instrument preset uses a Saturation, so no preset needs more
  columns.

Tests:
- Unit saturation-view: 14 new tests.
- Counter-check: without taking out the DC, exactly the DC test fails.
- e2e 34 (atx): ALL PASSED, 137 checks, one new for type, lever and
  drive with undo.
- Gate (atx): CHECK EXIT 0, 289 files, 4431 tests.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -47,7 +47,7 @@ npm run build        # writes dist/ and regenerates index.html; the folder is wh
 The browser suite and the three example games need a machine with a GPU:
 
 ```bash
-node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1968 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1969 checks (GM_STEPS=20 for one step)
                                                    #   every tool signs in first – see the Accounts page
 node tools/seed-jumprun.mjs && node tools/smoke-jumprun.mjs
 node tools/seed-tetris.mjs && node tools/smoke-tetris.mjs