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 }));

src/cli.js

@@ -0,0 +1,43 @@
+'use strict';
+
+const USAGE = `usage: sporel-mapper <subcommand> [args...]
+
+Subcommands:
+  encode <atlas_idx> <tile_id> [rotation]
+      Print the packed-u32 GID for the given components.
+
+  decode <gid>
+      Print the (atlas, tile, rotation) components of a GID.
+
+  build <spec.txt> <out.json> [--atlas-dir <path> ...]
+      Compile a text-DSL map spec into a v2 map JSON file.
+
+  inspect <map.json> [--atlas-dir <path> ...]
+      Print a human-readable stats report for a v2 map.
+
+Options:
+  -h, --help    Show this message.
+`;
+
+function run(argv, { out, err }) {
+  if (argv.length === 0) {
+    err.write(USAGE);
+    return 1;
+  }
+  const [head, ...rest] = argv;
+  if (head === '-h' || head === '--help') {
+    out.write(USAGE);
+    return 0;
+  }
+  const handlers = {
+    // populated in subsequent tasks
+  };
+  const handler = handlers[head];
+  if (!handler) {
+    err.write(`unknown subcommand: ${head}\n${USAGE}`);
+    return 1;
+  }
+  return handler.run(rest, { out, err });
+}
+
+module.exports = { run };

tests/cli.test.js

@@ -0,0 +1,46 @@
+'use strict';
+
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const { run } = require('../src/cli');
+
+class Sink {
+  constructor() { this.chunks = []; }
+  write(s) { this.chunks.push(s); }
+  get text() { return this.chunks.join(''); }
+}
+
+test('cli: no args prints usage to stderr, exit 1', () => {
+  const out = new Sink(); const err = new Sink();
+  const code = run([], { out, err });
+  assert.equal(code, 1);
+  assert.match(err.text, /usage/i);
+  assert.match(err.text, /encode/);
+  assert.match(err.text, /decode/);
+  assert.match(err.text, /build/);
+  assert.match(err.text, /inspect/);
+  assert.equal(out.text, '');
+});
+
+test('cli: --help prints usage to stdout, exit 0', () => {
+  const out = new Sink(); const err = new Sink();
+  const code = run(['--help'], { out, err });
+  assert.equal(code, 0);
+  assert.match(out.text, /usage/i);
+  assert.equal(err.text, '');
+});
+
+test('cli: -h alias for --help', () => {
+  const out = new Sink(); const err = new Sink();
+  const code = run(['-h'], { out, err });
+  assert.equal(code, 0);
+  assert.match(out.text, /usage/i);
+});
+
+test('cli: unknown subcommand goes to stderr, exit 1', () => {
+  const out = new Sink(); const err = new Sink();
+  const code = run(['frobnicate'], { out, err });
+  assert.equal(code, 1);
+  assert.match(err.text, /unknown subcommand/i);
+  assert.match(err.text, /frobnicate/);
+});