Help: Fn+h lists the keys of the screen you're on, and no screen names its keys any more (#69)

Fn+h on any screen, text fields included, and ? outside Text Entry, open a
panel over the content area: the screen's own keys, then the ones that work
everywhere. Every App declares its keys for the state it is in (pages,
viewers, dialogs and text fields answer for themselves); the App manager
opens the panel and takes every key while it is open.

About 30 hint lines are gone, from every App. What stays on a screen is
state. The first-start Setup keeps its hints and teaches the key; a device
set up before gets one Toast, once. The guide and the FAQ open with it.
`key help` over the consoles.

476 host tests (8 new). Checked on the device with key help and screenshots:
the Launcher, all nine Apps and several of their states. 8.5 KB of flash and
40 bytes of static RAM. Decisions Q196 to Q203 in docs/milestones/U1.md.

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

lib/core/src/key_help.h

@@ -0,0 +1,92 @@
+#pragma once
+
+#include <algorithm>
+#include <string>
+#include <vector>
+
+#include "key_event.h"
+
+// The help panel (issue #69, docs/milestones/U1.md): Fn+h on any screen lists the keys that work
+// there. Every App says what its keys are in the state it's in; nothing on a screen names keys.
+namespace roro {
+
+// One line of the panel: a key (or keys) and what it does. A line with no action is a heading.
+struct KeyHelp {
+    const char* keys;
+    const char* action;
+};
+
+namespace help {
+
+// Lists that several screens share.
+inline void list(std::vector<KeyHelp>& out, const char* enter = "open") {
+    out.push_back({"; .", "up, down"});
+    out.push_back({"Enter", enter});
+}
+inline void dialog(std::vector<KeyHelp>& out) {
+    out.push_back({", /", "the other answer"});
+    out.push_back({"Enter", "choose it"});
+    out.push_back({"`", "cancel"});
+}
+inline void textEntry(std::vector<KeyHelp>& out, const char* enter = "save") {
+    out.push_back({"Enter", enter});
+    out.push_back({"`", "cancel"});
+    out.push_back({"Del", "delete backwards"});
+    out.push_back({"Fn , /", "move the cursor"});
+    out.push_back({"opt ' e", "an accent: \xC3\xA9"});
+}
+// What works on every screen: the end of every panel.
+inline void everywhere(std::vector<KeyHelp>& out) {
+    out.push_back({"Everywhere", nullptr});
+    out.push_back({"`", "back"});
+    out.push_back({"Fn `", "home, the Launcher"});
+    out.push_back({"; . , /", "arrows (Fn+ while typing)"});
+    out.push_back({"Fn h  ?", "these keys (? not typing)"});
+}
+
+}  // namespace help
+
+// The panel itself: what it lists, and how far it's scrolled. The keys it takes while open are the
+// arrows; any other key closes it, and none reaches the App.
+class HelpModel {
+   public:
+    explicit HelpModel(int visibleRows = 8) : visible_(visibleRows) {}
+
+    void open(const std::string& title, std::vector<KeyHelp> rows) {
+        title_ = title;
+        rows_ = std::move(rows);
+        top_ = 0;
+        open_ = true;
+    }
+    void close() {
+        open_ = false;
+        rows_.clear();
+        rows_.shrink_to_fit();  // nothing is kept while it's closed
+    }
+    bool isOpen() const { return open_; }
+
+    void onKey(const KeyEvent& e) {
+        int last = std::max(0, static_cast<int>(rows_.size()) - visible_);
+        switch (e.key) {
+            case Key::Up: top_ = std::max(0, top_ - 1); break;
+            case Key::Down: top_ = std::min(last, top_ + 1); break;
+            case Key::Left: top_ = std::max(0, top_ - visible_); break;
+            case Key::Right: top_ = std::min(last, top_ + visible_); break;
+            default: close(); break;
+        }
+    }
+
+    const std::string& title() const { return title_; }
+    const std::vector<KeyHelp>& rows() const { return rows_; }
+    int top() const { return top_; }
+    int visibleRows() const { return visible_; }
+
+   private:
+    std::string title_;
+    std::vector<KeyHelp> rows_;
+    int top_ = 0;
+    int visible_;
+    bool open_ = false;
+};
+
+}  // namespace roro