chore: gitignore temp cargo target dirs

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

Author: snomiao

.gitignore

@@ -60,3 +60,5 @@ bun.lockb
 test-results/
 .playwright-mcp/
 tmp/
+rs/target-bootstrap/
+rs/target-v3/