ci: pin packageManager for pnpm/action-setup; vendor fiber tarball for CI installs

reference/ is gitignored but package.json points at the locally-built fiber
alpha.3 tarball — CI can't install without it. Vendored (1.3MB) as a stopgap;
unwinds when a published v10 alpha lands on npm (tracked in docs/UPSTREAM.md).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: DennisSmolek

.gitignore

@@ -1,6 +1,11 @@
 node_modules/
 dist/
-reference/
+# reference/* (not reference/) so the tarball exception below can apply —
+# git cannot re-include files inside a directory-level ignore.
+reference/*
+# TEMP (docs/UPSTREAM.md #1): CI needs the locally-built fiber tarball until a
+# published v10 alpha exists on npm; drop the exception when swapping to it.
+!reference/react-three-fiber-10.0.0-alpha.3.tgz
 .DS_Store
 *.local
 test-results/

package.json

@@ -3,6 +3,7 @@
   "private": true,
   "version": "0.0.0",
   "type": "module",
+  "packageManager": "pnpm@10.33.0",
   "scripts": {
     "dev": "vite",
     "build": "tsc --noEmit && vite build",