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/runtime/heap.c

@@ -210,6 +210,29 @@ void* _calloc(size_t count, size_t size) {
     return ptr;
 }
 
+
+// Returns the usable payload size of an allocation made by this heap.
+size_t _malloc_usable_size(const void* ptr) {
+    if (!ptr) return (size_t)-1;
+
+    HeapLock();
+    HEAP_SEGMENT* seg = SegmentForPointer((void*)ptr);
+    if (!seg) {
+        HeapUnlock();
+        return (size_t)-1;
+    }
+
+    HEAP_BLOCK* block = PayloadBlock((void*)ptr);
+    if (block->Magic != USER_HEAP_MAGIC || block->Free) {
+        HeapUnlock();
+        return (size_t)-1;
+    }
+
+    size_t size = block->Size;
+    HeapUnlock();
+    return size;
+}
+
 // Implements the realloc operation.
 void* _realloc(void* ptr, size_t newSize) {
     if (!ptr) return _malloc(newSize);