Add a read-only win32 context over a file descriptor

vmie_win32_open_ro_fd maps the RAM backing file PROT_READ (via
gpa_from_ro_fd) and does NOT bootstrap; the caller passes a cr3 to each
read call. The read surfaces work this way - sections / functions /
imports / exports / callgraph / hooks, gva_read_text, and the generic
gva_* and scanner surfaces via vmie_win32_mem() - while any write returns
-1 because the mapping is read-only. The RW twin vmie_win32_open_fd and
host_bootstrap are unchanged.

src/engine/win32/host.c

@@ -34,6 +34,21 @@ vmie_win32* vmie_win32_open_fd(int fd, uint64_t low) {
     return v;
 }
 
+/* Read-only context: no bootstrap, the cr3 is supplied per read call. The mem is
+ * mapped PROT_READ (gpa_from_ro_fd), so every write through it returns -1. */
+__attribute__((cold))
+vmie_win32* vmie_win32_open_ro_fd(int fd, uint64_t low) {
+    vmie_win32* v = calloc(1, sizeof *v);
+    if (!v) {
+        return NULL;
+    }
+    if (gpa_from_ro_fd(&v->mem, fd, low)) {
+        free(v);
+        return NULL;
+    }
+    return v;
+}
+
 __attribute__((cold))
 void vmie_win32_close(vmie_win32* v) {
     if (!v) {