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.

src/cli.js

@@ -30,7 +30,7 @@ function run(argv, { out, err }) {
     return 0;
   }
   const handlers = {
-    // populated in subsequent tasks
+    encode: require('./commands/encode'),
   };
   const handler = handlers[head];
   if (!handler) {

src/commands/encode.js

@@ -0,0 +1,32 @@
+'use strict';
+
+const { encodeGid } = require('../gid');
+
+const USAGE = 'usage: sporel-mapper encode <atlas_idx> <tile_id> [rotation]\n';
+
+function parseIntStrict(s, field) {
+  if (!/^-?\d+$/.test(s)) {
+    throw new RangeError(`${field} '${s}' is not an integer`);
+  }
+  return parseInt(s, 10);
+}
+
+function run(args, { out, err }) {
+  if (args.length < 2 || args.length > 3) {
+    err.write(USAGE);
+    return 1;
+  }
+  try {
+    const atlas = parseIntStrict(args[0], 'atlas');
+    const tile = parseIntStrict(args[1], 'tile');
+    const rotation = args.length === 3 ? parseIntStrict(args[2], 'rotation') : 0;
+    const gid = encodeGid(atlas, tile, rotation);
+    out.write(`${gid}\n`);
+    return 0;
+  } catch (e) {
+    err.write(`error: ${e.message}\n`);
+    return 1;
+  }
+}
+
+module.exports = { run };