Add Porta424 marketing site with live Web Audio + Web MIDI demo

.github/workflows/pages.yml

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+name: Deploy demo site to GitHub Pages
+
+# Publishes the static marketing site + live demo in web/ to GitHub Pages.
+# Enable once under Settings → Pages → Source: GitHub Actions.
+
+on:
+  push:
+    branches: [main]
+    paths:
+      - 'web/**'
+      - '.github/workflows/pages.yml'
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+# Allow one concurrent deployment, cancelling in-progress runs.
+concurrency:
+  group: pages
+  cancel-in-progress: true
+
+jobs:
+  deploy:
+    runs-on: ubuntu-latest
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/configure-pages@v5
+      - name: Upload web/ as Pages artifact
+        uses: actions/upload-pages-artifact@v3
+        with:
+          path: web
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v4

web/README.md

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+# Porta424 — Marketing site & live demo
+
+A single-page marketing site for **PortaDSPKit / Porta424** with an interactive,
+in-browser recreation of the 4-track cassette portastudio. The tape-character DSP
+runs live via the **Web Audio API** (an `AudioWorklet`), and the synth + tape
+knobs are playable over the **Web MIDI API**.
+
+Zero dependencies, no build step — plain HTML/CSS/ES modules, matching the
+product's "zero dependencies" ethos.
+
+![Hero](assets/screenshot-hero.png)
+![Interactive deck](assets/screenshot-deck.png)
+
+## Run locally
+
+Because ES modules and `AudioWorklet` require `http(s)://` (not `file://`), serve
+the folder with any static server:
+
+```bash
+cd web
+python3 -m http.server 8000   # then open http://localhost:8000
+# or:  npx serve .
+```
+
+Click **POWER** first — browsers require a user gesture before audio can start.
+
+## What the demo does
+
+| Area | Implementation |
+| --- | --- |
+| Tape character | `js/tape-worklet.js` — an `AudioWorkletProcessor` porting the C++ `wow_flutter`, `saturation`, `hiss` and `dropouts` modules (modulated fractional delay line, `tanh` soft clip, additive colored noise, smoothed random dropouts). |
+| Head bump / bandwidth | Native `BiquadFilterNode`s (peaking ~80 Hz Q 1.4, lowpass 1–20 kHz). |
+| 6-channel mixer | Per channel: trim, 3-band EQ, pan, fader, record-arm, source select — each its own Web Audio node chain into a shared mix bus. |
+| Sources | Built-in subtractive **synth** (Web MIDI + on-screen/computer keyboard), **microphone** (`getUserMedia`), and **audio file** upload (drag-and-drop or picker). |
+| Presets | `js/presets.js` mirrors the 5 factory presets from `PortaDSPFactoryPresets.swift`; values are converted back to UI/audio params with the same math as `DSPState.swift`. |
+| Metering / transport | `AnalyserNode` RMS VU meters, animated reels, LED tape counter. |
+
+### Graceful degradation
+
+- No `AudioWorklet` → falls back to a `WaveShaper` (tanh) + noise buffer for hiss
+  (wow/flutter/dropouts disabled); the on-screen badge reports this.
+- No Web MIDI → the on-screen and computer keyboards still play the synth.
+- Honors `prefers-reduced-motion`.
+
+## File layout
+
+```
+web/
+├── index.html            # all marketing sections + demo shell
+├── css/styles.css        # retro design system (palette from RetroTheme.swift)
+├── js/
+│   ├── main.js           # boots engine, builds deck, wires controls
+│   ├── audio-engine.js   # Web Audio graph (channels, tape chain, master, meters)
+│   ├── tape-worklet.js   # AudioWorkletProcessor (tape DSP)
+│   ├── synth.js          # polyphonic subtractive synth
+│   ├── midi.js           # Web MIDI access, notes + CC mapping
+│   ├── ui-controls.js    # accessible Knob & Fader components
+│   └── presets.js        # factory presets + DSPState mapping
+└── assets/               # favicon + screenshots
+```
+
+## Deployment
+
+`.github/workflows/pages.yml` publishes `web/` to GitHub Pages on every push to
+`main`. Enable it under **Settings → Pages → Source: GitHub Actions**.