feat: discover printer by MAC (survives DHCP IP changes)

Add discover_printer(), _mac_of(), _port_open(), _sweep_arp(), _cache_ip()
with PRINTER_MAC/PRINTER_SUBNET/CACHE_FILE config + unit tests. Also adds
conftest.py to work around broken symlink that blocked pytest collection.

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

pos_briefing.py

@@ -7,6 +7,7 @@ import os
 import re
 import json
 import socket
+import subprocess
 import requests
 import yfinance as yf
 from datetime import datetime
@@ -31,6 +32,14 @@ REMOTE_HOST   = config["remote_host"]
 REMOTE_PASS   = config["remote_pass"]
 PRINTER_IP    = config["printer_ip"]
 PRINTER_PORT  = int(config.get("printer_port", 9100))
+
+from pathlib import Path
+
+PRINTER_MAC    = config.get("printer_mac", "").lower()
+PRINTER_SUBNET = config.get("printer_subnet", "192.168.0")
+CACHE_DIR      = Path.home() / ".cache" / "pos-briefing"
+CACHE_FILE     = CACHE_DIR / "last_ip"
+
 EASTERN_TZ    = ZoneInfo("America/New_York")
 
 # TM-m30 80mm paper, Font B — column width measured from test print
@@ -437,6 +446,63 @@ def build_receipt(articles, zfs, reddit, grafana, weather, finance):
     return p.output
 
 
+def _mac_of(ip):
+    subprocess.run(["ping", "-c1", "-W1", ip], capture_output=True)
+    out = subprocess.run(["ip", "neigh", "show", ip],
+                         capture_output=True, text=True).stdout
+    m = re.search(r"lladdr ([0-9a-f:]{17})", out)
+    return m.group(1).lower() if m else None
+
+
+def _port_open(ip, port, timeout=3):
+    try:
+        with socket.create_connection((ip, port), timeout=timeout):
+            return True
+    except OSError:
+        return False
+
+
+def _sweep_arp():
+    """Concurrent ping-prime the /24, then return {mac: ip} from the neigh table."""
+    procs = [subprocess.Popen(["ping", "-c1", "-W1", f"{PRINTER_SUBNET}.{i}"],
+                              stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
+             for i in range(1, 255)]
+    for p in procs:
+        p.wait()
+    out = subprocess.run(["ip", "neigh", "show"],
+                         capture_output=True, text=True).stdout
+    found = {}
+    for line in out.splitlines():
+        m = re.match(r"([\d.]+).*lladdr ([0-9a-f:]{17})", line)
+        if m:
+            found[m.group(2).lower()] = m.group(1)
+    return found
+
+
+def _cache_ip(ip):
+    CACHE_DIR.mkdir(parents=True, exist_ok=True)
+    CACHE_FILE.write_text(ip)
+
+
+def discover_printer():
+    """Return the printer's current IP, resolved by MAC. Raise if not found."""
+    seeds = []
+    if CACHE_FILE.exists():
+        seeds.append(CACHE_FILE.read_text().strip())
+    if PRINTER_IP:
+        seeds.append(PRINTER_IP)
+    for ip in seeds:
+        if ip and _mac_of(ip) == PRINTER_MAC and _port_open(ip, PRINTER_PORT):
+            _cache_ip(ip)
+            return ip
+    if PRINTER_MAC:
+        ip = _sweep_arp().get(PRINTER_MAC)
+        if ip and _port_open(ip, PRINTER_PORT):
+            _cache_ip(ip)
+            return ip
+    raise RuntimeError(f"printer-not-found mac={PRINTER_MAC}")
+
+
 def print_receipt(raw_bytes):
     try:
         with socket.create_connection((PRINTER_IP, PRINTER_PORT), timeout=10) as sock: