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/Hero.tsx

@@ -64,8 +64,8 @@ export const Hero: React.FC = () => {
             />
           </div>
           <div className="rack-hint">
-            drag to rotate · scroll to nudge · <kbd>⇧</kbd> for fine control · double-click to
-            reset · arrow keys work too
+            drag to rotate · scroll to nudge · <kbd>⇧</kbd>+scroll for fine control ·
+            double-click to reset · arrow keys work too
           </div>
         </div>
       </div>