Add build subcommand: DSL spec to v2-map JSON file

Reads the DSL file, parses to AST, loads all --atlas-dir paths into
a registry, runs the builder, and writes pretty-printed JSON. The
output filename's basename becomes the map id when the DSL has no
'id' directive. Parser and builder errors are surfaced as
'<file>:<line>: <message>'. Also reorders the duplicate-atlas warning
so the atlas_id appears before the word 'duplicate'.

tests/build.test.js

@@ -0,0 +1,98 @@
+'use strict';
+
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const os = require('node:os');
+const path = require('node:path');
+const { run } = require('../src/cli');
+
+class Sink {
+  constructor() { this.chunks = []; }
+  write(s) { this.chunks.push(s); }
+  get text() { return this.chunks.join(''); }
+}
+
+const FIX_ATLAS = path.join(__dirname, 'fixtures', 'atlases');
+const FIX_DSL = path.join(__dirname, 'fixtures', 'dsl');
+
+function tmpOut(name) {
+  return path.join(os.tmpdir(), `${name}.json`);
+}
+
+test('build: minimal DSL → valid v2 JSON file', () => {
+  const out = new Sink(); const err = new Sink();
+  const outPath = tmpOut('minimal');
+  try {
+    const code = run(['build', path.join(FIX_DSL, 'minimal.txt'), outPath, '--atlas-dir', FIX_ATLAS], { out, err });
+    assert.equal(code, 0, err.text);
+    const json = JSON.parse(fs.readFileSync(outPath, 'utf8'));
+    assert.equal(json.schema_version, 2);
+    assert.equal(json.id, 'minimal');           // derived from out filename
+    assert.deepEqual(json.size, { w: 4, h: 4 });
+    assert.deepEqual(json.atlases, ['atlas_a']);
+    assert.equal(json.layers.surface.tiles.length, 16);
+  } finally {
+    if (fs.existsSync(outPath)) fs.unlinkSync(outPath);
+  }
+});
+
+test('build: full DSL with id directive overrides filename-derived id', () => {
+  const out = new Sink(); const err = new Sink();
+  const outPath = tmpOut('full');
+  try {
+    const code = run(['build', path.join(FIX_DSL, 'full.txt'), outPath, '--atlas-dir', FIX_ATLAS], { out, err });
+    assert.equal(code, 0, err.text);
+    const json = JSON.parse(fs.readFileSync(outPath, 'utf8'));
+    assert.equal(json.id, 'full_demo');
+    assert.deepEqual(Object.keys(json.layers).sort(), ['surface', 'wall']);
+    assert.equal(json.roof.length, 16);
+  } finally {
+    if (fs.existsSync(outPath)) fs.unlinkSync(outPath);
+  }
+});
+
+test('build: out-of-bounds DSL reports file:line on stderr, exit 1', () => {
+  const out = new Sink(); const err = new Sink();
+  const outPath = tmpOut('err');
+  const inPath = path.join(FIX_DSL, 'error_oob.txt');
+  const code = run(['build', inPath, outPath, '--atlas-dir', FIX_ATLAS], { out, err });
+  assert.equal(code, 1);
+  assert.match(err.text, /error_oob\.txt:4:/);
+  assert.match(err.text, /out of bounds/);
+  // Output file must NOT exist
+  assert.equal(fs.existsSync(outPath), false);
+});
+
+test('build: missing --atlas-dir for DSL that needs atlases → error', () => {
+  const out = new Sink(); const err = new Sink();
+  const outPath = tmpOut('noatlas');
+  const code = run(['build', path.join(FIX_DSL, 'minimal.txt'), outPath, '--atlas-dir', '/no/such/dir'], { out, err });
+  assert.equal(code, 1);
+  assert.match(err.text, /error/i);
+  if (fs.existsSync(outPath)) fs.unlinkSync(outPath);
+});
+
+test('build: insufficient positional args prints usage, exit 1', () => {
+  const out = new Sink(); const err = new Sink();
+  const code = run(['build', 'only_one_arg.txt'], { out, err });
+  assert.equal(code, 1);
+  assert.match(err.text, /usage: sporel-mapper build/);
+});
+
+test('build: duplicate atlas across --atlas-dir paths warns on stderr but exits 0', () => {
+  // Use both fixture dirs (atlases + atlases-dup)
+  const FIX_DUP = path.join(__dirname, 'fixtures', 'atlases-dup');
+  const out = new Sink(); const err = new Sink();
+  const outPath = tmpOut('dup');
+  try {
+    const code = run(
+      ['build', path.join(FIX_DSL, 'minimal.txt'), outPath, '--atlas-dir', FIX_ATLAS, '--atlas-dir', FIX_DUP],
+      { out, err }
+    );
+    assert.equal(code, 0, err.text);
+    assert.match(err.text, /warning.*atlas_a.*duplicate/i);
+  } finally {
+    if (fs.existsSync(outPath)) fs.unlinkSync(outPath);
+  }
+});