Add self-check via control-api (self_check.methods)

control-api is hosted outside the cloud and validators reach it directly,
so it sees the floating IP as the connection's source address. New open
route GET /api/v1/agents/{id}/observed-ip returns that address (taken only
from the TCP peer; forwarding headers are ignored so a validator cannot
forge it).

The agent gets self_check.methods, a priority-ordered list of ip_echo
(unchanged) and control_api; the default stays [ip_echo]. The self-check
passes when any method confirms the address; the next method is tried on
no answer and on a mismatch. Each method has its own timeout so a hung
first method cannot starve the fallback, and control_api uses a new TCP
connection per call (a connection opened before the floating IP was
attached would keep reporting the old address).

Also: docker agent template/env, example config, docs, plan in
docs/changes, e2e script switch E2E_SELF_CHECK_METHODS, rebuilt
bin/control-api and bin/validator-agent.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

scripts/run-local-e2e.sh

@@ -110,6 +110,9 @@ control_api_url: "http://127.0.0.1:28080"
 poll_interval_seconds: 1
 self_check:
   timeout_seconds: 5
+  # ip_echo by default; E2E_SELF_CHECK_METHODS="[control_api]" runs the same
+  # scenario through control-api's observed-ip route (loopback sees 127.0.0.1).
+  methods: ${E2E_SELF_CHECK_METHODS:-[ip_echo]}
   # Points at the local httpstub's /ip echo route rather than a real
   # internet IP-echo service — self-check only produces a meaningful
   # signal here because the "floating IP" under test (127.0.0.1) is