fix(sacn): validate universe range (1..63999), add boundary tests

Universe 0 and values >63999 are reserved per E1.31 spec and must be rejected.
Priority is deliberately not validated (compared relatively in later tasks only).

Test coverage:
- test_universe_zero_rejected: rejects universe 0
- test_universe_too_high_rejected: rejects universe 64000
- test_universe_is_read: continues to accept universe 63999

src/net/nl_e131.c

@@ -47,8 +47,16 @@ bool nl_e131_parse(const uint8_t *buf, size_t len, nl_e131_packet_t *out)
     if (buf[112] & 0x80u)    /* Preview-Daten duerfen laut E1.31 nicht ausgegeben werden */
         return false;
 
-    out->universe = be16(buf + 113);
-    out->priority = buf[108];
+    /* Universe must be in range 1..63999; 0 and values >63999 are reserved. */
+    uint16_t universe = be16(buf + 113);
+    if (universe == 0u || universe > 63999u)
+        return false;
+
+    out->universe = universe;
+    out->priority = buf[108];  /* taken as-is without range check; priority is only compared
+                                * relatively in later tasks, never against absolute bounds.
+                                * strict validation would silently drop light data from slightly
+                                * non-conformant senders, worse than accepting 0..255 */
     out->sequence = buf[111];
     out->terminated = (buf[112] & 0x40u) != 0u;
     out->slot_count = (uint16_t)(pvc - 1u);