Add decode subcommand

Accepts decimal or 0x-hex GID input. gid=0 is rendered as the
literal 'empty'; non-zero values print the three components in a
parse-stable space-separated key=value form.

tests/decode.test.js

@@ -0,0 +1,61 @@
+'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('decode: 196 prints (0,12,1)', () => {
+  const out = new Sink(); const err = new Sink();
+  const code = run(['decode', '196'], { out, err });
+  assert.equal(code, 0);
+  assert.equal(out.text, 'atlas=0 tile=12 rotation=1\n');
+});
+
+test('decode: 0 prints "empty"', () => {
+  const out = new Sink(); const err = new Sink();
+  const code = run(['decode', '0'], { out, err });
+  assert.equal(code, 0);
+  assert.equal(out.text, 'empty\n');
+});
+
+test('decode: hex (0xc4) parses to 196', () => {
+  const out = new Sink(); const err = new Sink();
+  const code = run(['decode', '0xc4'], { out, err });
+  assert.equal(code, 0);
+  assert.equal(out.text, 'atlas=0 tile=12 rotation=1\n');
+});
+
+test('decode: non-zero with tile=0 prints components (not "empty")', () => {
+  // gid=4 = (0<<24) | (0<<4) | (1<<2) → atlas=0 tile=0 rot=1
+  const out = new Sink(); const err = new Sink();
+  const code = run(['decode', '4'], { out, err });
+  assert.equal(code, 0);
+  assert.equal(out.text, 'atlas=0 tile=0 rotation=1\n');
+});
+
+test('decode: 16777232 from demo_v2 wall layer → (1,1,0)', () => {
+  const out = new Sink(); const err = new Sink();
+  const code = run(['decode', '16777232'], { out, err });
+  assert.equal(code, 0);
+  assert.equal(out.text, 'atlas=1 tile=1 rotation=0\n');
+});
+
+test('decode: no args prints usage, exit 1', () => {
+  const out = new Sink(); const err = new Sink();
+  const code = run(['decode'], { out, err });
+  assert.equal(code, 1);
+  assert.match(err.text, /usage: sporel-mapper decode/);
+});
+
+test('decode: bogus arg prints error, exit 1', () => {
+  const out = new Sink(); const err = new Sink();
+  const code = run(['decode', 'xyz'], { out, err });
+  assert.equal(code, 1);
+  assert.match(err.text, /error/i);
+});