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/platform/https_get.h

@@ -16,6 +16,10 @@ namespace roro {
 // release. Redirects aren't followed: the device only goes where it was told to (Q163).
 class HttpsGet {
    public:
+    // A TLS connection to Gitea peaks at about 52 KB of heap (measured: the same with or without
+    // checking the certificate); with Q86's 20 KB to spare, it starts with at least this much free.
+    static constexpr size_t kNeedFree = 80 * 1024;
+
     explicit HttpsGet(net::User user) : tls_(user) {}
     ~HttpsGet() { close(); }