docs: document preferences dialog, default-unchecked layers, robust routing + turn-by-turn

README.md

@@ -66,7 +66,8 @@ static/data/
 
 The frontend loads `/data/_index.json`, then fetches each layer's GeoJSON and
 renders it as a toggleable overlay with a legend (top-right). Every layer can be
-shown/hidden independently.
+shown/hidden independently. **By default no layers are shown on load** — toggle the
+ones you want from the legend.
 
 ## OSM Data-Dump Ingestion (`osm-dump.mjs`)
 
@@ -221,6 +222,12 @@ The matcher:
   matches still hit (e.g. `philbrick` finds `Philbrook`).
 - Ranks name matches above address matches above description matches.
 
+## Preferences & Units
+
+Click **⚙ Preferences** in the top bar to choose **Metric** or **Imperial** units.
+Your choice (and the granular unit override) is saved to `localStorage`, so it
+persists across reloads and is shared by the Directions panel.
+
 ## Directions & Routing
 
 When a dump layer is present (it carries the routable graph), a **Directions**
@@ -235,6 +242,13 @@ button appears in the top bar. Opening it lets you:
   large one for long routes), plus a **travel mode** (Drive or Walk).
 - See the route's **distance** and an **estimated travel time** (computed from
   the mode's average speed).
+- **Turn-by-turn directions** (*Head north*, *Turn right*, *Arrive at destination*,
+  …) are generated for **Drive** mode and listed beneath the result; a live
+  progress bar shows while the route computes.
+- **Robustness:** if A/B are far from the routable road network, or the destination
+  is disconnected / fully blocked by avoided categories, the app reports *why* and
+  still draws the **closest reachable** portion of a route (flagged as a partial
+  path) instead of failing silently.
 
 Routing is done **entirely in the browser** (`src/lib/routing.ts`): an
 A*-search over the graph built from the OSM dump. Point A/B are snapped to the
@@ -248,6 +262,10 @@ const r = route(graph, [latA, lonB], [latB, lonB], pois, [
   { category: 'amenity', value: 'school', radiusM: 300, block: true }
 ]);
 // r.found, r.path ([[lat,lon],...]), r.distanceM
+// r.partial (best-effort partial route when the destination is unreachable),
+// r.fromSnapM / r.toSnapM (distance from A/B to nearest road),
+// r.reason (human explanation when no full route exists),
+// r.turns (turn-by-turn steps for driving mode)
 ```
 
 ## Deployment