Add CLI dispatcher and bin entry

Top-level run(argv,{out,err}) accepts an injectable output sink so
handlers can be unit-tested without spawning a subprocess. Handler
map is empty for now; subcommands are wired in as they land.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/cli.js

@@ -0,0 +1,43 @@
+'use strict';
+
+const USAGE = `usage: sporel-mapper <subcommand> [args...]
+
+Subcommands:
+  encode <atlas_idx> <tile_id> [rotation]
+      Print the packed-u32 GID for the given components.
+
+  decode <gid>
+      Print the (atlas, tile, rotation) components of a GID.
+
+  build <spec.txt> <out.json> [--atlas-dir <path> ...]
+      Compile a text-DSL map spec into a v2 map JSON file.
+
+  inspect <map.json> [--atlas-dir <path> ...]
+      Print a human-readable stats report for a v2 map.
+
+Options:
+  -h, --help    Show this message.
+`;
+
+function run(argv, { out, err }) {
+  if (argv.length === 0) {
+    err.write(USAGE);
+    return 1;
+  }
+  const [head, ...rest] = argv;
+  if (head === '-h' || head === '--help') {
+    out.write(USAGE);
+    return 0;
+  }
+  const handlers = {
+    // populated in subsequent tasks
+  };
+  const handler = handlers[head];
+  if (!handler) {
+    err.write(`unknown subcommand: ${head}\n${USAGE}`);
+    return 1;
+  }
+  return handler.run(rest, { out, err });
+}
+
+module.exports = { run };