feat(scripts): add internal route-planner CLI for high-score walks

Picks a near-optimal walking route from a start to an end point within a
time budget, maximising the postbox points along the way. Uses straight-
line distances + configurable speed, beam-search orienteering, and reads
from live Firestore or a local JSON export.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: code418

functions/package.json

@@ -10,7 +10,8 @@
     "start": "npm run shell",
     "deploy": "firebase deploy --only functions",
     "logs": "firebase functions:log",
-    "test": "npm run build && nyc mocha lib/test/test.index.js --reporter spec"
+    "test": "npm run build && nyc mocha lib/test/test.index.js --reporter spec",
+    "plan-route": "npm run build && node lib/scripts/plan_route.js"
   },
   "engines": {
     "node": "22"