Edit a Compressor in a tab of its own, with knobs and levels (#378)

A Compressor had only its number rows in the node. Its numbers are
easier to set by feel on a dial, and what it does is easier to judge
when the level going in and the level going out are seen side by side.

- Knob is a new widget in the toolkit: a slider bent into a circle,
  its name above and a number field below. A drag up or down turns
  it, Shift a tenth as far, a notch of the wheel a fiftieth of the
  turn, a double-click sets the default, the number takes a typed
  value. A taper gives the low end more of the turn.
- The Compressor's tab opens on a double-click, one per node, like
  the Parametric EQ's. It has a knob for each of the seven numbers,
  greyed while its input is linked.
- Its curve shows level in to level out from -60 to 0 dB. The point
  where it bends sets the threshold, the end of the curve sets the
  ratio that ends there.
- Its levels show the last 4 s on both sides of the node. Where it
  takes more than 0.5 dB away, the stretch below the level coming in
  is tinted. The makeup is left out of that.
- The Compressor is tapped like the EQ. SpectrumTaps listens to more
  than one node at once and reads the loudest sample of the last
  25 ms on each side.
- The node tabs share one map in the editor (nodeTabs), so a third
  kind needs no new plumbing.

Findings:

- CI task 614 for 23922ee was red on the memory check of the stream:
  the tab's renderer grew by 94 MB with nothing decoded. Play mode
  made garbage of its own and was read before a collection. Both
  readings in play now come after one, and the check compares the
  growth streamed with the growth decoded (3.7 MB against 59.8 MB).
- The ratio handle sat on the right edge of the picture, where a
  widget's hit area ends. The curve is inset by the grab radius.

Tests:

- Gate (melektaus-atx): 282 files, 4338 tests (17 new)
- e2e 34 (melektaus-atx): ALL PASSED, 116 checks (6 new)
- Counter-checks: an inverted ratio search, stop() letting go of
  every node and a drag without its least step each fail exactly
  their test.
- Looked at the tab in a screenshot: a blip at -4.4 dB came out at
  -16.7 dB, the gap tinted, the threshold line at -24.

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

src/viewport/SoundLibrary.ts

@@ -58,7 +58,7 @@ import {
   releaseEnvelope,
   stopProcessor,
 } from './fxNodes';
-import { type Spectrum, SpectrumTaps } from './spectrum';
+import { type Levels, type Spectrum, SpectrumTaps } from './spectrum';
 import { decodeSpan, StreamFeed, StreamFile } from './streaming';
 import { wake } from './wake';
 
@@ -620,19 +620,24 @@ export class SoundLibrary implements AudioSink {
   }
 
   /**
-   * Listens to the sound in front of the node `id` (`tapId`) and behind it (#374): a Parametric
-   * EQ's tab while it is seen. `spectrum` reads it, `unlisten` lets it go.
+   * Listens to the sound in front of the node `id` (`tapId`) and behind it (#374): the tab of a
+   * Parametric EQ or a Compressor (#378) while it is seen. `spectrum` and `nodeLevels` read it,
+   * `unlisten` lets it go.
    */
   listen(id: string): void {
     this.spectra.listen(id);
   }
 
-  unlisten(): void {
-    this.spectra.stop();
+  unlisten(id: string): void {
+    this.spectra.stop(id);
   }
 
-  spectrum(): Spectrum | null {
-    return this.spectra.read();
+  spectrum(id: string): Spectrum | null {
+    return this.spectra.read(id);
+  }
+
+  nodeLevels(id: string, seconds: number): Levels | null {
+    return this.spectra.levels(id, seconds);
   }
 
   /** The node listened to, the taps playing, and how many of them feed an analyser (test hook). */