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'.

src/commands/build.js

@@ -0,0 +1,94 @@
+'use strict';
+
+const fs = require('node:fs');
+const path = require('node:path');
+const { parseDsl } = require('../dsl-parser');
+const { loadAtlasDirs } = require('../atlas-loader');
+const { buildMap } = require('../builder');
+
+const USAGE = 'usage: sporel-mapper build <spec.txt> <out.json> [--atlas-dir <path> ...]\n';
+
+function parseArgs(args) {
+  const positional = [];
+  const atlasDirs = [];
+  let i = 0;
+  while (i < args.length) {
+    const a = args[i];
+    if (a === '--atlas-dir') {
+      if (i + 1 >= args.length) {
+        throw new Error(`--atlas-dir requires a path`);
+      }
+      atlasDirs.push(args[i + 1]);
+      i += 2;
+    } else {
+      positional.push(a);
+      i += 1;
+    }
+  }
+  return { positional, atlasDirs };
+}
+
+function run(args, { out, err }) {
+  let parsed;
+  try {
+    parsed = parseArgs(args);
+  } catch (e) {
+    err.write(`error: ${e.message}\n${USAGE}`);
+    return 1;
+  }
+  if (parsed.positional.length !== 2) {
+    err.write(USAGE);
+    return 1;
+  }
+  const [specPath, outPath] = parsed.positional;
+
+  let dslText;
+  try {
+    dslText = fs.readFileSync(specPath, 'utf8');
+  } catch (e) {
+    err.write(`error: cannot read ${specPath}: ${e.message}\n`);
+    return 1;
+  }
+
+  let ast;
+  try {
+    ast = parseDsl(dslText);
+  } catch (e) {
+    const line = e.line || '?';
+    err.write(`${specPath}:${line}: ${e.message}\n`);
+    return 1;
+  }
+
+  let registry;
+  try {
+    const result = loadAtlasDirs(parsed.atlasDirs);
+    registry = result.registry;
+    for (const w of result.warnings) {
+      err.write(`warning: ${w}\n`);
+    }
+  } catch (e) {
+    err.write(`error: ${e.message}\n`);
+    return 1;
+  }
+
+  const defaultId = path.basename(outPath, path.extname(outPath));
+  let map;
+  try {
+    map = buildMap(ast, registry, { defaultId });
+  } catch (e) {
+    const line = e.line || '?';
+    err.write(`${specPath}:${line}: ${e.message}\n`);
+    return 1;
+  }
+
+  try {
+    fs.writeFileSync(outPath, JSON.stringify(map, null, 2) + '\n');
+  } catch (e) {
+    err.write(`error: cannot write ${outPath}: ${e.message}\n`);
+    return 1;
+  }
+
+  return 0;
+}
+
+module.exports = { run };