Updates from Gitea, step 3: install from Gitea, and IRC steps aside for it

A release downloads straight into the inactive slot through the existing
install path: the signature is checked after 160 bytes, before anything
is written, the hash at the end. Tried on the device against the real
server: a download cut short, a flipped byte in the signature and one in
the image are each refused with the running firmware untouched; the real
v0.10.0 installed, restarted, and confirmed itself on Probation.

A TLS connection to Gitea peaks at about 52 KB of heap whether or not the
certificate is verified. With IRC connected (66 KB free) a check left 3 KB
and a download 836 bytes. A check, list or install a person asks for now
makes IRC step aside (holdForUpdate) and come back after: the lowest free
heap during a full download with IRC connected is 38 KB. The daily check
never interrupts IRC; with IRC up it waits. A TLS connection starts with
80 KB free (it was 55).

Debug Builds get test knobs: update probe <host>, update damage cut|flip,
update pretend <version>.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

src/apps/irc_app.cpp

@@ -16,7 +16,7 @@ const char* statusText(Status s) {
         case Status::Connecting: return "connecting...";
         case Status::Registering: return "logging in...";
         case Status::Online: return "";
-        case Status::Paused: return "paused (Wi-Fi monitoring)";
+        case Status::Paused: return "paused (Wi-Fi monitoring or an update)";
         case Status::Retrying: return "retrying soon";
     }
     return "";