Add encode subcommand

Wraps gid.encodeGid, prints the packed-u32 as decimal on stdout,
emits usage on stderr on bad arity, and a labelled error on
out-of-range or non-integer input.

tests/encode.test.js

@@ -0,0 +1,54 @@
+'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('encode: (0,12,1) prints 196 with trailing newline, exit 0', () => {
+  const out = new Sink(); const err = new Sink();
+  const code = run(['encode', '0', '12', '1'], { out, err });
+  assert.equal(code, 0);
+  assert.equal(out.text, '196\n');
+  assert.equal(err.text, '');
+});
+
+test('encode: rotation defaults to 0', () => {
+  const out = new Sink(); const err = new Sink();
+  const code = run(['encode', '0', '12'], { out, err });
+  assert.equal(code, 0);
+  assert.equal(out.text, '192\n');
+});
+
+test('encode: no args prints usage to stderr, exit 1', () => {
+  const out = new Sink(); const err = new Sink();
+  const code = run(['encode'], { out, err });
+  assert.equal(code, 1);
+  assert.match(err.text, /usage: sporel-mapper encode/);
+});
+
+test('encode: too many args prints usage, exit 1', () => {
+  const out = new Sink(); const err = new Sink();
+  const code = run(['encode', '0', '0', '0', '0'], { out, err });
+  assert.equal(code, 1);
+  assert.match(err.text, /usage/);
+});
+
+test('encode: out-of-range tile prints error, exit 1', () => {
+  const out = new Sink(); const err = new Sink();
+  const code = run(['encode', '0', '99999999', '0'], { out, err });
+  assert.equal(code, 1);
+  assert.match(err.text, /error: tile/);
+});
+
+test('encode: non-integer arg prints error, exit 1', () => {
+  const out = new Sink(); const err = new Sink();
+  const code = run(['encode', '0', 'abc', '0'], { out, err });
+  assert.equal(code, 1);
+  assert.match(err.text, /error/i);
+});