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/lib/messages.js

@@ -0,0 +1,19 @@
+// Message type + target constants. Single source of truth — popup,
+// service worker, and offscreen document all import from here so we
+// can't drift on string typos.
+
+export const TARGETS = Object.freeze({
+  SW:        'sw',
+  OFFSCREEN: 'offscreen',
+  POPUP:     'popup',
+});
+
+export const TYPES = Object.freeze({
+  PLAY:           'PLAY',
+  PAUSE:          'PAUSE',
+  SET_VOLUME:     'SET_VOLUME',
+  GET_STATE:      'GET_STATE',
+  STATE_CHANGED:  'STATE_CHANGED',
+  METADATA_UPDATED: 'METADATA_UPDATED',
+  ERROR:          'ERROR',
+});