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>

deploy/docker/docker-compose.yml

@@ -100,6 +100,7 @@ services:
       VALIDATOR_AGENT_CONTROL_API_URL: "${VALIDATOR_AGENT_CONTROL_API_URL:-http://control-api:8080}"
       VALIDATOR_AGENT_POLL_INTERVAL_SECONDS: "${VALIDATOR_AGENT_POLL_INTERVAL_SECONDS:-5}"
       VALIDATOR_AGENT_SELF_CHECK_TIMEOUT_SECONDS: "${VALIDATOR_AGENT_SELF_CHECK_TIMEOUT_SECONDS:-10}"
+      VALIDATOR_AGENT_SELF_CHECK_METHODS: "${VALIDATOR_AGENT_SELF_CHECK_METHODS:-[ip_echo]}"
       VALIDATOR_AGENT_HTTPS_TIMEOUT_SECONDS: "${VALIDATOR_AGENT_HTTPS_TIMEOUT_SECONDS:-10}"
       VALIDATOR_AGENT_ICMP_TIMEOUT_SECONDS: "${VALIDATOR_AGENT_ICMP_TIMEOUT_SECONDS:-5}"
       VALIDATOR_AGENT_ICMP_COUNT: "${VALIDATOR_AGENT_ICMP_COUNT:-3}"