watcher: follow a watched name's CNAME for port and TLS checks (closes #203)

When a watched name's nameservers answer with a CNAME and no address,
the DNS check follows every target they gave with ResolveIPAddresses
and saves all addresses found as cnameAddresses in the hostname state,
so nameservers disagreeing on the target do not change them between
checks. Port and TLS checks use them. A change, also from or to none,
is notified as a CNAME address change; the first check from a state
file without them sends none. When a target cannot be followed, or
none of the name's nameservers answered, the last check's addresses
are kept. The domain check now runs the hostname check for the apex
instead of a copy of it.

Model: opus-5-5

README.md

@@ -121,14 +121,26 @@ notification endpoint set, changes show only on the dashboard; see
       failed on it, and answers differently is reported on the check where it
       answers. If a pair agrees again and later disagrees, the alert is sent
       again.
+    - **CNAME address change**: The addresses at the end of a name's CNAME chain
+      differ from those of the previous check. They are found when its
+      nameservers answer with a CNAME and no address; a name that answers with
+      an address has none. A change from or to no addresses is sent too, as when
+      a name moves between A records and a CNAME. Nothing is sent when the
+      previous addresses were kept because a chain could not be followed or none
+      of the name's nameservers answered. The first check after loading a state
+      file without `cnameAddresses` sends nothing: it saves the addresses it
+      finds for the next check to compare.
 
 ### TCP Port Monitoring
 
 - For every configured domain and hostname, constructs a deduplicated list of
   the IPv4 and IPv6 addresses in the A and AAAA records its authoritative
-  nameservers returned. A CNAME is not followed: a name whose CNAME points into
-  another zone usually has no addresses here, so its ports and certificate are
-  not checked.
+  nameservers returned. When they returned a CNAME and no address, the CNAME
+  chain is followed and the addresses at its end are used, and a change in those
+  is notified as a CNAME address change. When the nameservers gave different
+  CNAME targets, each is followed and the addresses of all are used. When a
+  chain cannot be followed, or none of the name's nameservers answered, the
+  addresses the last check found at its end are used.
 - Checks TCP connectivity on ports **80** and **443** for each IP address.
 - Every **1 hour** by default, re-checks all ports.
 - Any change in port availability triggers a notification:
@@ -176,6 +188,8 @@ includes:
 - **DNS NS changes**: Which domain, which nameservers were added/removed.
 - **NS address changes**: Which domain, which nameserver, its old and new
   addresses.
+- **CNAME address changes**: Which hostname, the old and new addresses at the
+  end of its CNAME chain.
 - **NS query failures**: Which nameserver failed, error type (timeout, SERVFAIL,
   REFUSED, network error), which hostname/domain affected.
 - **NS recoveries**: Which nameserver recovered, which hostname/domain.
@@ -420,9 +434,11 @@ This approach ensures:
 - Ability to detect split-horizon or inconsistent responses across authoritative
   servers.
 
-CNAME chains are followed (with a depth limit to prevent loops) only to find the
-addresses of nameservers. A watched name's records are stored as its nameservers
-return them, CNAME included, without following it.
+A watched name's records are stored as its nameservers return them, CNAME
+included. When they return a CNAME and no address, the chain of every CNAME
+target they gave is followed (with a depth limit to prevent loops) to the A and
+AAAA records at its end, and the port and TLS checks use those addresses.
+Nameservers' addresses are also found by following CNAME chains.
 
 Sending a notification or a Sentry report is the one use of the system's
 resolver: the HTTP client looks up the webhook's or Sentry's host name with it.
@@ -469,6 +485,7 @@ merged view, to enable inconsistency detection.
                     "lastChecked": "2026-02-19T12:00:00Z"
                 }
             },
+            "cnameAddresses": [],
             "lastChecked": "2026-02-19T12:00:00Z"
         }
     },
@@ -515,6 +532,14 @@ certificate entry whose TLS connection or handshake failed likewise has status
 resolves to. A state file without it loads, and the next check fills it in
 without a notification.
 
+`cnameAddresses` lists the sorted addresses at the end of the chain of every
+CNAME target a hostname's nameservers gave, found when they answered with a
+CNAME and no address; it is empty when they answered with an address. When a
+chain cannot be followed, or none of the name's nameservers answered, the
+previous check's list is kept, or `null` when no earlier check saved one. A
+state file without it loads, and the first check after that saves it without a
+notification.
+
 A port entry in the older format, with one `hostname` instead of the `hostnames`
 list, loads as a list of that one name.
 
@@ -672,7 +697,9 @@ docker run -d \
     - Port and TLS checks use the IP addresses found by the DNS phase that
       immediately precedes them. When that phase cannot find a name's
       nameservers at all, the addresses an earlier check saved for the name are
-      used.
+      used. When it cannot follow a name's CNAME chain, or none of the name's
+      nameservers answered, the addresses an earlier check found at the end of
+      the chain are used.
 4. **On change detection**: Send notifications to all configured endpoints,
    update in-memory state, persist to disk.
 5. **Shutdown**: The watcher stops checking and saves the final state to disk,