Add an Acid instrument node for TB-303 style bass lines (#421)

The instruments could only come near a 303 with a Chip, a Filter and
two envelopes. The biquad filter does not saturate, every note strikes
anew, and there was no slide and no accent.

- Acid is a new instrument node that plays a saw or a square through a
  four-pole ladder low-pass with tanh saturation, oversampled twice.
  Knobs: cutoff, resonance, env mod, decay, accent, tuning and slide
  time.
- An accent comes from the velocity, from 0.9 on. It is louder, opens
  the filter further with a shorter decay, and accents close behind
  each other build up.
- Two notes that touch or overlap slide. Each note stays a voice of
  its own, so the scheduler is unchanged. The notes of a line share
  one synth in the worklet, and the new voice takes it over from its
  first sample on.
- The node sets its own volume, so it needs no envelope behind it.
- The graph shows it as a device with the filter curve at rest and at
  the top of the sweep.
- A preset Acid Bass puts an Acid in front of a short echo.

Tests:
- tsc, eslint, Prettier clean
- Unit: 14 new tests in acid.test.ts for pitch, low-pass, resonance,
  sweep, accent, slide and the handover between two voices.
- e2e 34 on melektaus-atx: ALL PASSED, 148 checks (3 new). The
  handover sits on sample 4800 and jumps by 0.0012, of three notes
  only the second slides.
- Without the handover, or with a slide that strikes anew, the unit
  tests are red.
- Gate on melektaus-atx: tsc, eslint, Prettier clean, 4753 of 4754
  tests green. The red one was sound-library.test.ts, which did not
  know the new slid count in the stats. Fixed, and the file alone
  is green again with 128 tests. No second full gate.
- Not checked: the sound by ear, slide at the keyboard, a bake with
  Acid, the CPU load of many notes.

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, 2062 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 2065 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