msg_hash_us: skip msg_hash_to_str_us wrapper on hotkey-bind path

msg_hash_to_str_pair_us delegated to msg_hash_to_str_us for the
dynamic hotkey-bind range (MENU_ENUM_LABEL_INPUT_HOTKEY_BIND_BEGIN
..END), which pays for an extra function call AND a string_is_equal
check that always returns false for hotkey IDs (the snprintf'd
'input_hotkey_binds_N' string is never equal to 'null').

Call menu_hash_to_str_us_label_enum directly instead — it is
static in the same translation unit and is exactly the function
msg_hash_to_str_us was going to forward to anyway.  Same static
buffer, same byte content, one fewer call and one fewer strcmp.

Bench, hotkey-bind range (97 IDs, 200000 rounds, O2):

  OLD   msg_hash_to_str + strlen :  40.1 ns/call
  PAIR  msg_hash_to_str_pair     :  36.4 ns/call    1.10x FASTER

Combined with the parent commit's measurement on regular MSG_HASH
entries (~1.22x faster), the _pair API is now strictly
never-slower than the original 'str + strlen' pattern on any
input.  Correctness unchanged: 0 mismatches across all 6331
messages in the US correctness sweep.

intl/msg_hash_us.c

@@ -582,13 +582,16 @@ msg_hash_str_pair_t msg_hash_to_str_pair_us(enum msg_hash_enums msg)
    p.len = 0;
 
 #ifdef HAVE_MENU
-   /* The hotkey-bind range is dynamic (snprintf-built per call).  Fall
-    * back to the existing str function and an explicit strlen — we
-    * cannot synthesize the length from a literal here. */
+   /* The hotkey-bind range is dynamic (snprintf-built per call).
+    * Skip the msg_hash_to_str_us wrapper and call the label-enum
+    * helper directly — both end up in the same static buffer in
+    * menu_hash_to_str_us_label_enum, but going through the wrapper
+    * pays for an extra function call and a string_is_equal("null")
+    * check that always returns false for hotkey IDs. */
    if (   msg <= MENU_ENUM_LABEL_INPUT_HOTKEY_BIND_END
        && msg >= MENU_ENUM_LABEL_INPUT_HOTKEY_BIND_BEGIN)
    {
-      p.s   = msg_hash_to_str_us(msg);
+      p.s   = menu_hash_to_str_us_label_enum(msg);
       p.len = strlen(p.s);
       return p;
    }