perf(R): optimize ARM architecture checks and add documentation

Performance improvements:
- Check isX64R first to short-circuit when R is ARM64 (correct setup)
- Only check ARM architecture when R fails (x64 R on ARM always crashes)
- Avoids FFI overhead for all successful R checks

Documentation:
- Explains why kernel32.dll FFI is needed and safe
- Links to validation repo and Microsoft API docs

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

Co-Authored-By: Claude <[email protected]>

Author: cderv

src/core/knitr.ts

@@ -82,7 +82,7 @@ function checkWindowsArmR(platform: string | undefined): void {
 
   const isX64R = platform.includes("x86_64") || platform.includes("i386");
 
-  if (isWindowsArm() && isX64R) {
+  if (isX64R && isWindowsArm()) {
     throw new WindowsArmX64RError(
       "x64 R detected on Windows ARM.\n\n" +
         "x64 R runs under emulation and is not reliable for Quarto.\n" +
@@ -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.");

src/core/windows.ts

@@ -166,6 +166,17 @@ export async function safeWindowsExec(
 
 // Detect Windows ARM hardware using IsWow64Process2 API
 // Returns true if running on ARM64 hardware (even from x64 Deno under emulation)
+//
+// Background: Deno.build.arch reports the architecture Deno was compiled for,
+// not the actual hardware architecture. When x64 Deno runs on ARM64 under
+// WOW64 emulation, Deno.build.arch still reports "x86_64".
+//
+// Solution: Use Windows IsWow64Process2 API which returns the native machine
+// architecture regardless of emulation. This is a standard Windows API function
+// available since Windows 10 (kernel32.dll is always present on Windows).
+//
+// Reference: Validated approach from https://github.com/cderv/quarto-windows-arm
+// See: https://learn.microsoft.com/en-us/windows/win32/api/wow64apiset/nf-wow64apiset-iswow64process2
 export function isWindowsArm(): boolean {
   if (!isWindows) {
     return false;