perf(R): only check ARM architecture when R fails

x64 R on ARM always crashes, so checking in the success path is
unnecessary. This avoids FFI overhead for all successful R checks
(the common case: x64 R on x64 Windows, ARM64 R on ARM64 Windows).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: cderv

src/core/knitr.ts

@@ -131,7 +131,6 @@ export async function knitrCapabilities(rBin: string | undefined) {
         )
         : false;
 
-      checkWindowsArmR(caps.platform);
       return caps;
     } else {
       debug("\n++ Problem with results of knitr capabilities check.");