fix: direct-IP fallback when printer_mac unset; force utf-8 in bridge subprocess

Final-review hardening:
- discover_printer falls back to direct IP reachability when printer_mac is
  empty (restores legacy behavior; avoids permanent unreachable on misconfig).
  Keeps the MAC check when a MAC IS set so we never print to a wrong DHCP tenant.
- bridge subprocess decodes child stdout as utf-8 explicitly, so a LANG=C systemd
  locale can't UnicodeDecodeError on the success checkmark and mis-report error.
- add tests for both no-MAC fallback paths (5 passed).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

pos_briefing.py

@@ -498,6 +498,13 @@ def discover_printer():
         seeds.append(CACHE_FILE.read_text().strip())
     if PRINTER_IP:
         seeds.append(PRINTER_IP)
+    if not PRINTER_MAC:
+        # No MAC configured: fall back to direct IP reachability (legacy behavior).
+        for ip in seeds:
+            if ip and _port_open(ip, PRINTER_PORT):
+                _cache_ip(ip)
+                return ip
+        raise RuntimeError("printer-not-found (no printer_mac configured)")
     for ip in seeds:
         if ip and _mac_of(ip) == PRINTER_MAC and _port_open(ip, PRINTER_PORT):
             _cache_ip(ip)