add page cache with read-ahead and dirty writeback, file-backed and anonymous mmap with CoW, VAD-based virtual memory replacing the old user region model, PFN database with eviction and reclaim, file identity for cross-open section sharing, FAT32 path cache and per-file cluster chain cache, serial.sys for COM1 keyboard input, xHCI MSI with INTx and polled fallback, PciEnableMsiAuto, waitable input queue, KeWaitForAnyEvent, per-process thread info syscall, expanded kernel stats (PFN/PTE/VAD/file-cache/disk), pool allocation tags, async I/O user buffer locking, file flush on close, add docs.md.

code/user/apps/shell_win32/shell_win32.c

@@ -147,6 +147,8 @@ static int runCommand(const char* cmd) {
         WaitForSingleObject(pi.hProcess, INFINITE);
         if (pi.hThread && pi.hThread != INVALID_HANDLE_VALUE)
             CloseHandle(pi.hThread);
+        if (pi.hProcess && pi.hProcess != INVALID_HANDLE_VALUE)
+            CloseHandle(pi.hProcess);
         FlushConsoleInputBuffer(hIn);
     } else {
         print("shell: launch failed\n");