app: link.ra_source router identification + brand icons + DEV build variant

link.ra_source answers "who advertises IPv6 here, and which box is it":
RA source per network, MAC recovered from the modified-EUI-64 link-local
(privacy addresses reported as such, not guessed), vendor via a curated
OUI table, UPnP/SSDP M-SEARCH for the gateway's server banner + device
description (manufacturer/model/friendly name), and reverse DNS. All SSDP
responders are recorded so a rogue RA sender that isn't the gateway can
still be matched; the MAC accompanies every identity source as the hook
for future LLDP/mDNS cross-matching. UI gains a "Router / IPv6 advertiser"
panel.

Icons: branding adaptive icon converted to vector drawables (+ PNG
mipmaps, monochrome layer). The debug build is now a separate app —
applicationIdSuffix .dev, label "Echolot DEV", DEV-badged icon — so it
installs alongside a production build and can't be confused with it.

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

echolot-app/app/src/main/res/drawable/ic_launcher_background.xml

@@ -0,0 +1,17 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- SPDX-License-Identifier: GPL-3.0-or-later
+     Adaptive-icon background: the brand "tile" gradient (assets/branding). -->
+<vector xmlns:android="http://schemas.android.com/apk/res/android"
+    xmlns:aapt="http://schemas.android.com/aapt"
+    android:width="108dp" android:height="108dp"
+    android:viewportWidth="108" android:viewportHeight="108">
+    <path android:pathData="M0,0h108v108h-108z">
+        <aapt:attr name="android:fillColor">
+            <gradient android:startX="54" android:startY="0" android:endX="54" android:endY="108"
+                android:type="linear">
+                <item android:offset="0" android:color="#FF0E2433"/>
+                <item android:offset="1" android:color="#FF071522"/>
+            </gradient>
+        </aapt:attr>
+    </path>
+</vector>