Document the extension and add MIT LICENSE

User-verified that v0.1.0 works in Firefox. Closing out the work with
the documentation the AMO reviewer (and future-me) will want:

- LICENSE: MIT, 2026. Matches the vendored qrcode-generator's license.
- package.json: add license + author fields.
- README: add "What you see" UX section, supported-versions rationale,
  ASCII build-flow diagram, AMO publishing notes, permissions table.
- popup/popup.js: top-of-file block documenting the popup lifecycle and
  the dataflow from active-tab URL to rendered QR.
- scripts/build.mjs: top-of-file block explaining why dist/ is the
  single source of truth for what ships.

Closes dogcat epic firefox-share-as-qr-35mw and all 5 child tasks
(see .dogcats/issues.jsonl).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/build.mjs

@@ -1,3 +1,14 @@
+// Build entry point. Run via `bun run build` (or transitively via
+// `bun run lint` / `package` / `start`).
+//
+// Output: a self-contained `dist/` directory that web-ext consumes as its
+// sourceDir. The bundler inlines `vendor/qrcode.js` into the popup script
+// (via the ESM import in popup.js), so the published extension is one JS
+// file plus the static HTML/CSS/icon/license assets.
+//
+// Anything outside `dist/` never ships, so this script is the single source
+// of truth for "what is the extension."
+
 import { rm, cp } from "node:fs/promises";
 
 const DIST = "dist";