docs: design for HA/Google Home on-demand POS briefing trigger

Replaces the never-installed cron approach with an MQTT bridge mirroring the
lgtv pattern: Google Home button -> HA script/scene -> MQTT -> steel141 prints.
Adds MAC-based printer discovery (printer IP is DHCP-unstable) and a gemma4
summarizer swap. Includes scripts/dryrun.py (non-printing pipeline harness).

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

scripts/dryrun.py

@@ -0,0 +1,49 @@
+#!/usr/bin/env python3
+"""Dry-run harness: exercises the full pos_briefing pipeline WITHOUT printing.
+
+Monkeypatches print_receipt so nothing is sent to the TM-m30 over TCP. Reports
+which data sources succeeded/failed and the receipt byte length. Use to verify
+the script runs end-to-end on a host before scheduling it via cron.
+"""
+import os
+import sys
+
+sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+import pos_briefing as pb
+
+
+def fake_print_receipt(raw):
+    print(f"[dry-run] print_receipt() suppressed — {len(raw)} bytes would be sent "
+          f"to {pb.PRINTER_IP}:{pb.PRINTER_PORT}")
+    return True
+
+
+pb.print_receipt = fake_print_receipt
+
+
+def probe(name, fn):
+    try:
+        v = fn()
+        ok = v is not None and v != [] and v != {}
+        print(f"  {'OK ' if ok else 'NIL'} {name}: {repr(v)[:120]}")
+        return v
+    except Exception as e:
+        print(f"  ERR {name}: {type(e).__name__}: {e}")
+        return None
+
+
+print("=== data source probes ===")
+items, token = (pb.fetch_news() if True else (None, None))
+print(f"  news: {len(items) if items else 0} items, token={'yes' if token else 'no'}")
+zfs = probe("zfs", pb.get_zfs_status)
+reddit = probe("reddit", pb.get_reddit_top)
+grafana = probe("grafana", pb.get_grafana_metrics)
+weather = probe("weather", pb.get_weather)
+finance = probe("finance", pb.get_financial_snapshot)
+
+print("=== build + (suppressed) print ===")
+articles = pb.summarize_news(items, count=5, length="30-50 words") if items else []
+print(f"  articles summarized: {len(articles)}")
+raw = pb.build_receipt(articles, zfs, reddit, grafana, weather, finance)
+pb.print_receipt(raw)
+print("=== done (nothing printed) ===")