perf(scanner): stop redoing everything on every advertisement, plus map point events

Yes, the scanner was duplicating. refreshCategoryOptions(),
updateRadar() and pairedAdapter.notifyDataSetChanged() ran from
acceptSignal on every advertisement AND from pruneTask once a second
- the same three calls either way.

Worse, the whole persist path ran per advertisement, and ScanCallback
is delivered on the main looper: a hex dump of the advertisement,
lookupCompany(), get(), lookupOui(), then telemetry(),
setDetectedCategory(), recordObservation() and setVendor(). Eight
database operations on the UI thread per beacon, at
SCAN_MODE_LOW_LATENCY, times every transmitter in range. The radar
sweep re-posts itself every 33ms and was competing for that thread,
which is why it dragged.

A device already on the radar now only gets its RSSI and last-seen
stamp refreshed until PERSIST_INTERVAL_MS (3s) is up. A device's
first sighting still takes the full path and refreshes at once.

The radar view also allocated per frame at 30fps: a six-stop
RadialGradient plus two arrays, and a fresh ArrayList and Comparator
for the target sort. Both reused now. setTargets no longer
invalidates while scanning, since onDraw already re-posts itself.
Left setLayerType(SOFTWARE) alone - setShadowLayer only works on text
under hardware acceleration, so removing it would drop the ring glow.

Separately, history points now say why they exist. location_history
had no event column, so schema 16 adds one; connected() and
disconnected() stamp it and a plain position refresh leaves it null.
The popup shows a linked or broken-link line for the two states and
stays quiet for older rows that predate the column.

History points also render as the device's own marker at half size
rather than a plain circle, sharing one divIcon per device so a long
trail does not build a DOM node per point.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app/src/main/java/com/wytehat/btlogger/LocationPoint.java

@@ -1,8 +1,22 @@
 package com.wytehat.btlogger;
 
 public class LocationPoint {
+
+    /** The item linked to the phone at this position. */
+    public static final String EVENT_CONNECTED = "connected";
+
+    /** The link to the phone dropped at this position. */
+    public static final String EVENT_DISCONNECTED = "disconnected";
+
     public double latitude;
     public double longitude;
     public float accuracy;
     public long timestamp;
+
+    /**
+     * Why this point was recorded: EVENT_CONNECTED, EVENT_DISCONNECTED, or
+     * null for a plain position refresh while the link state was unchanged.
+     * Null for every row logged before the column existed.
+     */
+    public String event;
 }