SSH client: a terminal on another machine (#2)

The SSH App opens one session to a shell, over libssh (LibSSH-ESP32 5.10.0)
on mbedTLS. A server is trusted the first time on its fingerprint, and a
changed key is a warning with Cancel selected. The password is typed each
time and kept nowhere; or the device makes itself an Ed25519 key, whose
public half is shown, written to /ssh/id_ed25519.pub and printed by
`ssh status`.

lib/term is the terminal: what a shell, less, top, nano and vim send, with
sixteen colours, scroll regions, the alternate screen and 100 lines of
scrollback. Five text sizes with Ctrl and + or -, from 60x20 to 26x8, told
to the far end. The session goes on when the App is left; SSH shows in the
Status Bar. `ssh user@host` in the Shell opens the App.

Also:
- Keys that aren't characters carry Shift, Ctrl and Alt. The terminal needs
  it, and it makes Ctrl+Fn+up/down in a note and Shift+Tab in Gemini work
  from the real keyboard.
- IRC doesn't try to connect under 60 KB free: started with a session open,
  its TLS handshake took the heap down to 236 bytes.
- libssh's own curve25519 is left out of the build (scripts/libssh_filter.py):
  libsodium's has the same names.

Costs 292 KB of flash and about 50 KB of heap while a session is open; not
started under 75 KB free.

Docs: guide page, how-to, FAQ, home page, Status Bar, SD card folders, the
memory how-to, README, glossary, N1 notes with Q254 to Q266 and the checks.

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

lib/term/src/ssh_hosts.h

@@ -0,0 +1,49 @@
+#pragma once
+
+#include <cstdint>
+#include <string>
+#include <vector>
+
+// Who the SSH App connects to, and which servers it has met (issue #2, N1 Q262 and Q263): kept
+// in the device's settings as a few lines of text.
+namespace roro::term {
+
+struct SshTarget {
+    std::string user, host;
+    uint16_t port = 22;
+    std::string text() const;      // user@host, with :port when it isn't 22
+    std::string hostPort() const;  // host:port, always: what a host key is remembered under
+};
+// "user@host", "user@host:2222". "" or what is wrong with it.
+std::string parseSshTarget(const std::string& text, SshTarget& out);
+
+// Up to eight, the last used first; one a line.
+class SshHosts {
+   public:
+    static constexpr size_t kMax = 8;
+    explicit SshHosts(const std::string& stored = "");
+    const std::vector<std::string>& list() const { return hosts_; }
+    void used(const SshTarget& target);  // to the front, added if it's new
+    void remove(size_t index);
+    std::string stored() const;
+
+   private:
+    std::vector<std::string> hosts_;
+};
+
+// The fingerprint each server showed the first time: "host:port SHA256:...", one a line, sixteen
+// at most (the oldest goes).
+class SshKnownHosts {
+   public:
+    static constexpr size_t kMax = 16;
+    explicit SshKnownHosts(const std::string& stored = "");
+    std::string fingerprintOf(const std::string& hostPort) const;  // "" if never met
+    void remember(const std::string& hostPort, const std::string& fingerprint);
+    void forget(const std::string& hostPort);
+    std::string stored() const;
+
+   private:
+    std::vector<std::pair<std::string, std::string>> known_;
+};
+
+}  // namespace roro::term