Docs accuracy pass, guides, and a playable Web Audio synth demo

Docs/playground accuracy (from a full docs-vs-source audit):
- Playground: drag-feel select covers pickup/relative rotary modes, is
  disabled for faders, and generated code no longer emits props the
  target component does not accept
- API/README claims corrected: Shift-fine wheel scope, wheelStep and
  decimals defaults, trackInset row, data-part and ref-forwarding
  scope, VintageKnob readout exception, Meter peakHold tag, stale
  component lists, aria-valuetext wording, also-exported list

API consistency fixes surfaced by the audit and the tutorial:
- Keyboard gestures now fire onChangeStart (bracket parity with drag/
  wheel - fixes gesture-scoped undo patterns)
- name (hidden form input) added to Fader, LEDFader, ImageKnob
- PushButton gets a themed keyboard-only focus ring + data-focus-visible

New docs content:
- Getting Started: 7-step guide with live examples and a gotchas list
- Advanced guide: 8-step channel-strip tutorial ending in a live,
  themed, gesture-undoable strip with meter (plus headless demo)
- Synth section: playable Web Audio synth - osc/filter/env/LFO/master
  all dreamknob controls, momentary-pad keyboard, analyser-driven Meter

apps/docs/src/sections/Gallery.tsx

@@ -165,9 +165,9 @@ export const Gallery: React.FC = () => (
       <div className="section-kicker">Gallery</div>
       <h2 className="section-title">Every style in the studio</h2>
       <p className="section-sub">
-        Nine prebuilt skins — every color themeable per instance. All of them share the
-        same interaction core: rotary or drag adjustment, scroll-wheel nudge, keyboard,
-        fine control and double-click reset.
+        A full rack of prebuilt controls — every color themeable per instance. All of
+        them share the same interaction core: rotary or drag adjustment, scroll-wheel
+        nudge, keyboard, fine control and double-click reset.
       </p>
 
       <div className="grid">
@@ -341,7 +341,7 @@ export const Gallery: React.FC = () => (
           </div>
         </Card>
 
-        <Card title="Meter" desc="Read-only LED metering with peak hold — the display sibling of LEDFader." tag="<Meter peakHold />">
+        <Card title="Meter" desc="Read-only LED metering with peak hold — the display sibling of LEDFader." tag="<Meter peakHold={1200} />">
           <MeterDemo />
         </Card>