ext: S1 native-messaging spike (ADR 0003) + WebSocket transport

Spike S1 — run on the target machine through real snap confinement
(apparmor snap.firefox.firefox enforced; web-ext's direct-exec of the inner
binary bypasses it, so runs were forced through `snap run firefox`):

  - manifest in ~/.mozilla/native-messaging-hosts/  -> WORKS; host launched
    unconfined with real $HOME and real $XDG_RUNTIME_DIR, bound a socket in
    the real /run/user/<uid>. Corroborated by the machine's 1Password host.
  - ~/snap/firefox/common/.mozilla/native-messaging-hosts/  -> not read
  - /usr/lib/mozilla/native-messaging-hosts/               -> not read
  - flatpak path                                           -> N/A (snap Firefox)

Decision: WebSocket stays the default; native messaging is an opportunistic
upgrade taken only when its handshake succeeds. docs/05 §4 corrected in this
commit to point the snap manifest at ~/.mozilla and mark /usr/lib as
deb/tarball-only. ADR carries a self-contained reproduction; the scratch
harness has been removed.

transport/ (build order item 1):
  - types.ts        VeloxTransport interface + error taxonomy
  - rpc.ts          JSON-RPC id correlation, per-call deadline, AbortSignal
  - backoff.ts      exponential backoff with jitter
  - discovery.ts    52000-52016 scan ordering (last-good port first)
  - websocket.ts    scan -> session.hello -> auto-pair (token in
                    storage.local) -> reconnect; -32001 fatal, refused/
                    rate-limited pairing latches needsPairing (no retry storm);
                    a mid-handshake drop aborts hello immediately
  - native.ts       connectNative(); distinguishes "not installed" (fatal,
                    lets the picker fall through) from a crash (reconnect)
  - index.ts        createTransport() runtime picker + persisted Options override

Toolchain: package.json / tsconfig (strict) / vitest; webextension-polyfill
mocked. 38 tests, incl. the WS suite against a real loopback ws server.
No manifest.json yet, so CI's extension-lint guard stays a no-op.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_012Y9RU58hD1BuwP82DySUHk

extension/package.json

@@ -0,0 +1,27 @@
+{
+  "name": "@velox/extension",
+  "version": "0.0.0",
+  "private": true,
+  "description": "Velox Firefox extension (lane EXT). Collects URL + headers + cookies and hands downloads to veloxd; renders daemon state. Zero download logic.",
+  "type": "module",
+  "scripts": {
+    "typecheck": "tsc --noEmit",
+    "test": "vitest run",
+    "test:watch": "vitest",
+    "lint": "web-ext lint --source-dir ."
+  },
+  "dependencies": {
+    "webextension-polyfill": "^0.12.0"
+  },
+  "devDependencies": {
+    "@types/webextension-polyfill": "^0.12.1",
+    "@types/ws": "^8.5.12",
+    "typescript": "^5.6.0",
+    "vitest": "^2.1.0",
+    "web-ext": "^8.3.0",
+    "ws": "^8.18.0"
+  },
+  "engines": {
+    "node": ">=20"
+  }
+}