standalone: ship the headless engine as one bundleable ESM file

A no-build static site couldn't vendor BitGate the way it vendors a single
dependency: bare @bitgate/* specifiers don't resolve in a browser, and the
headless path spans two packages (the evaluator in core, the kind-1985 label
codec in nostr). The widget bundle solved this for the DOM path only.

Add @bitgate/standalone (packages/standalone): an entry re-exporting core +
nostr, esbuilt into a single self-contained dist/bitgate.js (~44 KB). It's
the same canonical code the npm packages ship, with specifiers inlined, and
dependency-free -- @bitgate/verify's @noble deps are intentionally not pulled
in, so the bundle carries no third-party code. Wired into `npm run build` via
build:standalone.

Verified: no bare imports in the output (self-contained), and a smoke test
composes a full label -> decision flow from the single entry. README Packages
table and headless section document it.

Addresses the "standalone ESM single-file" item of the Tessera integration
feedback (docs/TODO.md).

Co-Authored-By: Claude Opus 4.8 <[email protected]>
Claude-Session: https://claude.ai/code/session_01BXQCvJPA6sVSQ5j8uBd9bB

packages/standalone/package.json

@@ -0,0 +1,19 @@
+{
+  "name": "@bitgate/standalone",
+  "version": "0.1.0",
+  "private": true,
+  "type": "module",
+  "license": "GPL-3.0-or-later",
+  "description": "Headless BitGate engine + Nostr codecs bundled into a single self-contained ESM file for no-build static sites.",
+  "main": "src/index.js",
+  "scripts": {
+    "build": "node build.mjs"
+  },
+  "dependencies": {
+    "@bitgate/core": "*",
+    "@bitgate/nostr": "*"
+  },
+  "devDependencies": {
+    "esbuild": "^0.28.2"
+  }
+}