fix: fall back to arm64 binary on darwin when x64 binary is missing

Silicon Mac users running x64 Node (via Rosetta or nvm default) got a
404 because muapi-darwin-x86_64 was not in the v0.2.5 release. Fall back
to the arm64 binary on darwin so the install succeeds regardless of which
Node arch is active.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Anil-matcha

npm/scripts/install.js

@@ -22,7 +22,8 @@ const NAME_MAP = {
 };
 
 const key     = `${process.platform}-${process.arch}`;
-const binName = NAME_MAP[key];
+// On Apple Silicon, Node may report x64 when running under Rosetta — fall back to arm64.
+const binName = NAME_MAP[key] || (process.platform === "darwin" ? NAME_MAP["darwin-arm64"] : undefined);
 
 if (!binName) {
   console.warn(`[muapi] Unsupported platform: ${key}. Install via pip: pip install muapi-cli`);