feat: overhaul userspace heap and virtual memory

programs/man/syscalls.2

@@ -185,9 +185,24 @@
         void* montauk::alloc(uint64_t size);
 
 .B SYS_FREE (12)
-    Reserved (currently a no-op).
+    Release a complete mapping previously returned by SYS_ALLOC.
         void montauk::free(void* ptr);
 
+.B SYS_MMAP_ANON (168)
+    Reserve a zero-filled anonymous mapping with read/write/execute
+    protection flags. Pages are committed on first access. Writable
+    executable mappings are rejected.
+        void* mmap(void*, size_t, int, int, int, long);
+
+.B SYS_MUNMAP (169)
+    Release a page-aligned range. Partial unmap splits the VM area and
+    makes the virtual range reusable.
+        int munmap(void* addr, size_t length);
+
+.B SYS_MPROTECT (170)
+    Change read/write/execute permissions on an anonymous mapping.
+        int mprotect(void* addr, size_t length, int prot);
+
 .B SYS_MEMSTATS (67)
     Get kernel-wide physical memory usage (total/free/used bytes,
     page size).