feat: initial commit — RangerHQ Tuner v0.1.0 (Tier 1 MVP)

Chrome MV3 extension, browser-resident sibling to rangerhq-radio
(WP plugin). Plays SomaFM via the chrome.offscreen API + a source-
adapter pattern at src/sources/.

Architecture highlights:
- Audio runs in offscreen document — SW would get killed.
- Source-adapter pattern locks Tier 1 contract (RadioSource interface
  in src/sources/base-source.js). Adding a network = drop a file +
  register one line in src/sources/index.js.
- Vanilla JS, no build step. Pure ES modules.
- No telemetry, no third-party JS. Outbound only to somafm.com.
- Narrow permissions: offscreen + storage + somafm.com host_perms.
  No tabs, no <all_urls>, no webRequest.

22 files, ~30 min build following the saved plan at
~/.ranger-memory/projects/rangerhq-tuner-plan.md.

Tier 2 + Tier 3 (Web Store submission) not started.

src/background/service-worker.js

@@ -0,0 +1,39 @@
+// RangerHQ Tuner — Service Worker.
+// Job: open the offscreen audio document on demand and route messages
+// between the popup (UI) and the offscreen document (audio engine).
+// We hold no state here — Chrome will kill this worker at any time.
+
+const OFFSCREEN_PATH = 'src/offscreen/offscreen.html';
+
+async function ensureOffscreen() {
+  // hasDocument is the official MV3 way to check. Optional-chained
+  // for older Chrome safety (we still require 116+ via the manifest).
+  const exists = await chrome.offscreen.hasDocument?.();
+  if (exists) return;
+  await chrome.offscreen.createDocument({
+    url: OFFSCREEN_PATH,
+    reasons: ['AUDIO_PLAYBACK'],
+    justification: 'Persistent audio playback for internet radio streams.',
+  });
+}
+
+chrome.runtime.onMessage.addListener((msg, sender, sendResponse) => {
+  // Only react to messages targeted at the service worker.
+  if (msg.target !== 'sw') return;
+
+  (async () => {
+    try {
+      await ensureOffscreen();
+      // Re-route the message to the offscreen document.
+      const response = await chrome.runtime.sendMessage({
+        ...msg,
+        target: 'offscreen',
+      });
+      sendResponse(response);
+    } catch (err) {
+      sendResponse({ ok: false, error: err.message });
+    }
+  })();
+
+  return true; // keep the channel open for async sendResponse
+});