M1 step 4b: IRC Service on the device; 8-bit frame buffer

- IrcService: networking on its own task, TLS with the built-in CA
  bundle (or trust-on-first-use pinning when self-signed is allowed),
  plain TCP when TLS is off; connects only while Wi-Fi is Connected,
  marks pauses for Monitoring, reconnects with backoff, pings a quiet
  server, writes Logs, raises Notifications for Mentions
- Session: forget /quit once disconnected (it was handled every loop);
  the server Buffer never counts as unread (MOTD showed as [4])
- Status Bar: unread count
- Frame buffer 16 -> 8-bit colour (M1 Q46): min free heap with IRC on
  TLS went from 51 KB to 79 KB
- Serial: irc start / say / dump

Verified on the device against irc.libera.chat:6697: certificate
checked, joined #roro9stack-test, sent a message, quit cleanly.

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

src/ui/screen.h

@@ -9,7 +9,7 @@ namespace roro {
 // Composes the frame (Status Bar, foreground App, Toast) off-screen and pushes it to the display.
 class Screen {
    public:
-    bool begin();  // allocates the frame buffer (~64 KB)
+    bool begin();  // allocates the frame buffer (~32 KB at 8-bit colour)
     void render(AppManager& apps, const StatusInfo& status, const Toast* toast);
     Canvas& canvas() { return canvas_; }