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/src/index.js

@@ -0,0 +1,15 @@
+// @bitgate/standalone — the headless engine and the Nostr codecs it consumes,
+// as one bundleable entry.
+//
+// Bare specifiers (`@bitgate/core`) don't resolve in a browser, and the headless
+// path needs two packages (the evaluator in core, the kind-1985 label codec in
+// nostr). build.mjs esbuilds this entry into a single self-contained ESM file a
+// no-build static site can vendor the way it would vendor any one dependency —
+// the requirement the widget bundle already meets for the DOM path, met here for
+// the headless path.
+//
+// Both re-exported packages are dependency-free (@bitgate/verify, the only
+// package with third-party deps, is intentionally not pulled in), so the bundle
+// carries no external code.
+export * from "@bitgate/core";
+export * from "@bitgate/nostr";