fix: include darwin-arm64 native binary in npm tarball (Phase A hotfix)

The native .node binary was excluded from npm pack due to native/.gitignore
ignoring addon/. Add native/.npmignore (overrides .gitignore for npm) and
include native/addon in the files whitelist. Also improve the error message
in the native loader to list supported platforms and link to issues.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

native/.npmignore

@@ -0,0 +1 @@
+target/

package.json

@@ -25,6 +25,7 @@
     "pkg",
     "src/resources",
     "scripts/postinstall.js",
+    "native/addon",
     "package.json",
     "README.md"
   ],

packages/native/src/native.ts

@@ -33,10 +33,19 @@ function loadNative(): Record<string, unknown> {
   }
 
   const details = errors.map((e) => `  - ${e}`).join("\n");
+  const supportedPlatforms = [
+    "darwin-arm64",
+    "darwin-x64",
+    "linux-x64",
+    "linux-arm64",
+    "win32-x64",
+  ];
   throw new Error(
     `Failed to load gsd_engine native addon for ${platformTag}.\n\n` +
       `Tried:\n${details}\n\n` +
-      `Build with: npm run build:native -w @gsd/native`,
+      `Supported platforms: ${supportedPlatforms.join(", ")}\n` +
+      `If your platform is listed, try reinstalling: npm i -g gsd-pi\n` +
+      `Otherwise, please open an issue: https://github.com/gsd-build/gsd-2/issues`,
   );
 }