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

package.json

@@ -14,7 +14,8 @@
     "check:references": "node scripts/verify-no-reference-imports.mjs",
     "check:types": "tsc -p tsconfig.json --noEmit && tsc -p packages/widget/tsconfig.json --noEmit",
     "build:widget": "node packages/widget/build.mjs",
-    "build": "npm run build:types && npm run build:widget && npm run build:site",
+    "build:standalone": "node packages/standalone/build.mjs",
+    "build": "npm run build:types && npm run build:widget && npm run build:standalone && npm run build:site",
     "build:site": "node packages/site/build.mjs",
     "dev:site": "node packages/site/build.mjs && node packages/site/serve.mjs"
   },