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.

include/win32.h

@@ -92,6 +92,20 @@ vmie_win32* vmie_win32_open(const char* ram_path, uint64_t low);
  * NULL on dup/fstat/mmap failure. Free with vmie_win32_close(). */
 vmie_win32* vmie_win32_open_fd(int fd, uint64_t low);
 
+/* Open a READ-ONLY context over the RAM backing file `fd`, for a consumer that
+ * only reads. Unlike vmie_win32_open_fd this does NOT bootstrap (the magic-
+ * signature handshake needs RW): the caller supplies the address space by passing
+ * a cr3 to each read call. Every read API that takes a cr3 works - the section /
+ * function / import / export / callgraph / hook surfaces, gva_read_text, and the
+ * generic gva_* and scanner surfaces via vmie_win32_mem(); writes (gva_write) -1
+ * because the mapping is PROT_READ. proc_list/proc_modules are NOT available here:
+ * they need the bootstrap-recovered profile, which a read-only context does not
+ * carry. `fd` is BORROWED (dup()'d internally; the caller's fd stays valid after
+ * this call AND after vmie_win32_close()); an O_RDONLY fd is accepted. `low` is
+ * the below-4G split, as in vmie_win32_open_fd. Returns a new context, or NULL on
+ * dup/fstat/mmap failure. Free with vmie_win32_close(). */
+vmie_win32* vmie_win32_open_ro_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);

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) {