Add CLI dispatcher and bin entry

Top-level run(argv,{out,err}) accepts an injectable output sink so
handlers can be unit-tested without spawning a subprocess. Handler
map is empty for now; subcommands are wired in as they land.

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

bin/mapper.js

@@ -0,0 +1,5 @@
+#!/usr/bin/env node
+'use strict';
+
+const { run } = require('../src/cli');
+process.exit(run(process.argv.slice(2), { out: process.stdout, err: process.stderr }));