app: long runs that watch, and a relay for the port that keeps moving

Long mode starts listeners at t=0 and keeps them running past the
battery: a network-change watcher that finally fills networks[].changes[]
(defined since the schema's first draft, never populated), an RSSI log, a
ping series giving loss and jitter over minutes, and mDNS listening for
the whole window. This is the class of fault a short run cannot see - a
link that drops for four seconds between two probes is reported healthy
by both of them. run.mode records which question was asked, because
silence means different things in the two modes.

The adb relay replaces the retired beacon: AdbRelay watches adbd's own
mDNS with the resolve-once discipline the beacon learned the hard way
(resolving re-arms adbd and pops a notification), a foreground service
keeps it alive with the screen off, and the heartbeat re-posts the cached
endpoint rather than re-resolving. It exists because mDNS does not cross
subnets and the wireless-debug port rotates every few minutes.

Also records why LLDP/CDP cannot follow SSDP into long mode: both are raw
L2 frames, so they need CAP_NET_RAW - root tier, not app, and Shizuku's
shell user does not have it either.

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

docs/findings-registry.md

@@ -50,6 +50,13 @@ because it looks authoritative.
 | `connectivity.downstream_reorder` | low | Downstream packets arrive in a different order than they were sent. | — |
 | `connectivity.captive_portal` | medium | A captive portal is intercepting connectivity checks. | — |
 | `connectivity.no_internet` | high | Android's own connectivity checks fail on this network. | — |
+| `connectivity.link_flapping` | medium | A network dropped and came back one or more times during the run. | A momentary probe failure: the drop was watched happening, not inferred from silence. |
+
+`connectivity.link_flapping` is only reachable from a **long run** (`run.mode: "long"`,
+measurement-schema §3). It is derived from `networks[].changes[]` rather than from any test's
+evidence, because no one-shot probe can produce it: the probes before and after a four-second drop
+both succeed. The emitter escalates to *high* from three completed drop-and-return cycles, and
+requires the cycle to complete — a network switched off partway through a run is not flapping.
 
 ### mtu