node: split Running into Full/Degraded, add Failed health state

Determine node health at start completion instead of unconditionally
reaching Running. Zero transports up is now Failed (fatal): start()
tears down cleanly and returns an error, and the daemon exits. Any
configured optional child that failed to start - a transport beyond the
first, Nostr, mDNS, TUN, DNS, or a worker pool - leaves the node
Degraded but serving, with an operator warning naming what failed. All
configured children up is Full. A child the node was never asked to run
does not count against health.

The published NodeState gains Degraded and Failed variants, both visible
via control queries; Degraded is operational, Failed is not. The
lifecycle FSM gains the health states plus the PublishState action that
drives them - a health fork cannot be a single direct state write, which
is why the earlier commits deferred it to here.

Runtime child-exit health re-evaluation (a running child dying) is a
separate liveness-monitoring mechanism left for a follow-up; this commit
is start-time health only.

src/node/tests/mod.rs

@@ -30,6 +30,25 @@ pub(super) fn make_node() -> Node {
     make_node_with(Config::new())
 }
 
+/// A test node that reaches `Full` health on `start()`.
+///
+/// A default [`make_node`] configures no transports, so its `start()` now
+/// resolves to `NodeState::Failed` (zero transports up, design doc §9.1) and
+/// returns `NoOperationalTransports`. Lifecycle-state tests that need a running
+/// node build one with a single loopback UDP transport (ephemeral port) as the
+/// sole configured child — DNS disabled — so bring-up has exactly one
+/// configured child and it comes up (`Full`). Mirrors the udp config in
+/// `test_node_start_does_not_wait_for_nostr_relay_startup`.
+pub(super) fn make_healthy_node() -> Node {
+    let mut config = Config::new();
+    config.transports.udp = crate::config::TransportInstances::Single(crate::config::UdpConfig {
+        bind_addr: Some("127.0.0.1:0".to_string()),
+        ..Default::default()
+    });
+    config.dns.enabled = false;
+    make_node_with(config)
+}
+
 /// Build a test node from an explicit `Config`. Immutable state lives solely in
 /// the shared `NodeContext`, built once at construction — there is no
 /// post-construction field to poke, so set limits/config on the `Config` here.