Construct the memory source from a file descriptor

vmie_mem could only be built from a path. Add a second input: an
already-open file descriptor, dup()'d internally (borrowed - the
caller's fd stays valid, core owns and closes its copy).

  - vmie_mem_from_fd / vmie_mem_from_fd_segs   (read-write, as the path
    constructors: PROT_RW, MAP_SHARED)
  - vmie_mem_from_ro_fd / vmie_mem_from_ro_fd_segs   (read-only: map
    PROT_READ, mark the source ro; gpa_write/gva_write return -1, every
    read path is unchanged; accepts an O_RDONLY fd)
  - vmie_win32_open_fd   (the win32 context over an fd backing file)

Factor the mmap/validate tail and the single-low segment map out of the
path constructors into shared helpers, so the path and fd inputs go
through one mmap site and one map builder each.

include/win32.h

@@ -83,6 +83,15 @@ typedef struct {
  * failure. Free with vmie_win32_close(). */
 vmie_win32* vmie_win32_open(const char* ram_path, uint64_t low);
 
+/* As vmie_win32_open, but over an already-open file descriptor `fd` for the RAM
+ * backing file instead of a path. `fd` is BORROWED: it is dup()'d internally, so
+ * the context owns its own copy and the caller's `fd` stays open and valid after
+ * this call AND after vmie_win32_close(). `fd` must reference an mmap-able,
+ * read-write object (regular file / memfd_create / POSIX shm); its file position
+ * is not read or changed. Returns a new context (call host_bootstrap() next), or
+ * NULL on dup/fstat/mmap failure. Free with vmie_win32_close(). */
+vmie_win32* vmie_win32_open_fd(int fd, uint64_t low);
+
 /* Unmap, close, and free a context. Safe on NULL. After this, every pointer
  * into guest memory obtained through this context is invalid. */
 void vmie_win32_close(vmie_win32* v);