Gemini: read big pages from the card as you scroll (Q88)

A page larger than memory allows is now windowed instead of cut: one
pass over its file (cache or Saved Page) counts lines, indexes every
64th (offset, and the preformatted state there in bit 31: about 1 KB
for a 1 MB page) and loads the first window. Scrolling near either end
reads the next or previous window on the Gemini task; the line on top
of the screen stays put, the scrollbar follows the whole page, and Tab
at a window's edge pages on instead of wrapping. Window budgets count
the memory the old window gives back.

Display pages alternate between two cache files so the one on screen
is never overwritten by the next fetch; jobs use a third.

On the device, Cosmos with IRC connected: 226 of 419 lines at first,
then lines 192-419, back to 64 and 0 while scrolling. `key space` added
to the console's key command.

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

src/main.cpp

@@ -350,7 +350,7 @@ static const char* const kHelp =
     "gnss status | gnss restart | gnss track start|stop | gnss nmea on|off | gnss send <sentence without $ and checksum>\n"
     "crash             the last crash: firmware, reason, task, backtrace\n"
     "coredump erase    forget the core dump in flash\n"
-    "key <name|char>   press a key: up down left right select back home del tab, or one character\n"
+    "key <name|char>   press a key: up down left right select back home del tab space, or one character\n"
     "wifi status | wifi add <ssid><TAB><password>\n"
     "gemini get <url>  fetch a Gemini page and report header, size, certificate, heap\n"
     "irc start | irc stop | irc dump | irc say <buffer> <text>\n"
@@ -435,7 +435,9 @@ static void runCommand(String line) {
         Key key = k == "up" ? Key::Up : k == "down" ? Key::Down : k == "left" ? Key::Left
                 : k == "right" ? Key::Right : k == "back" ? Key::Back : k == "home" ? Key::Home
                 : k == "del" ? Key::Delete : k == "tab" ? Key::Tab : Key::Select;
-        KeyEvent ev = k.length() == 1 && k[0] > ' ' ? KeyEvent::character((unsigned char)k[0]) : KeyEvent::of(key);
+        KeyEvent ev = k == "space"                       ? KeyEvent::character(' ')
+                      : k.length() == 1 && k[0] > ' ' ? KeyEvent::character((unsigned char)k[0])
+                                                      : KeyEvent::of(key);
         if (!power->onKey(millis())) apps->handleKey(ev);
     }
     if (line.startsWith("wifi add ")) {  // wifi add <ssid>\t<password>: credentials never touch the repo