Notes: plain text notes on the SD card, with an editor that saves by itself (#19)

The Notes App lists the files of /notes by their first line, newest first:
n starts a note, Enter opens it, r renames its file, d deletes it after
asking, s sorts by name. A new note's file is named after its first line.

The editor wraps at spaces, 38 columns by 8 rows; Fn+arrows move through
the wrapped text, Ctrl+A and Ctrl+E go to the ends of the line. There is no
save key: the note is written five seconds after the last key, on Back, on
leaving the App, when the screen turns off and before the device powers
off. A save writes a temporary file and puts it in the note's place; a save
cut short is put back, or offered, the next time.

A note is up to 16 KB, held in one buffer reserved when it's opened: the
file is read straight into it and typing never makes it grow. A failed
allocation aborts on this device, and with IRC connected the largest free
block is about 31 KB: a first version that copied the note once on loading
restarted the device when a full note was opened with IRC connected.
Editing files of any size is #47.

The Storage App's text viewer gets `e`, which edits a text file up to 16 KB
with the same editor unless the file is read-only.

439 host tests. Checked on the device: docs/milestones/F1.md.

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

lib/notes/src/note_text.h

@@ -0,0 +1,81 @@
+#pragma once
+
+#include <cstddef>
+#include <cstdint>
+#include <string>
+#include <vector>
+
+namespace roro::notes {
+
+// The text of a note while it's edited (F1, Q144, Q145): UTF-8 held whole in memory, a cursor, and
+// the part of it on screen. Lines wrap at spaces, `cols` characters wide; a line owns the space or
+// the newline it ends with, so every byte of the text belongs to exactly one line. No index of
+// lines is kept (a note of newlines alone would need twice its size): where a line starts is
+// worked out from the start of its paragraph, which is never far.
+class NoteText {
+   public:
+    static constexpr size_t kMaxBytes = 16 * 1024;
+
+    // Room for a full note is reserved once, so typing never has to find a bigger block of memory:
+    // on the device a failed allocation is the end. The second form takes over a string the
+    // caller filled (and reserved): a note is never in memory twice.
+    NoteText(int cols, int rows);
+    NoteText(int cols, int rows, std::string&& text);
+
+    // Copied into the buffer already held. CRLF becomes LF (Q148). False, and nothing changes, if
+    // it's over kMaxBytes.
+    bool setText(const std::string& text);
+    const std::string& text() const { return text_; }
+    size_t cursor() const { return cursor_; }
+    uint32_t revision() const { return revision_; }  // changes with every edit: is it saved?
+
+    bool insert(uint32_t codePoint);         // false: the note is full
+    bool insertText(const std::string& s);   // all of it or nothing
+    void backspace();
+    void left();
+    void right();
+    void up();
+    void down();
+    void pageUp();
+    void pageDown();
+    void lineStart();
+    void lineEnd();
+    void toStart();
+    void toEnd();
+
+    // The screen, kept around the cursor: its rows (tabs as spaces, the ending newline left out),
+    // and where the cursor is on it, in rows and characters.
+    std::vector<std::string> rows();
+    int cursorRow();
+    int cursorCol();
+    int percent() const { return text_.empty() ? 0 : static_cast<int>(top_ * 100 / text_.size()); }
+
+    std::string firstLine() const;  // without its newline, for the title and the file's name
+
+   private:
+    size_t nextLine(size_t start) const;       // where the line after the one at `start` starts
+    size_t lineOf(size_t pos) const;           // the start of the line `pos` is on
+    size_t lastSpot(size_t start) const;       // the last place the cursor can be on that line
+    size_t atColumn(size_t start, int col) const;
+    int columnOf(size_t start, size_t pos) const;
+    bool hasLineAfter(size_t start) const;
+    void moved(bool keepGoal = false);
+    void follow();  // scrolls so the cursor is on screen
+    void dropCarriageReturns();
+
+    int cols_, rows_;
+    std::string text_;
+    size_t cursor_ = 0, top_ = 0;
+    int goal_ = -1;  // the column Up and Down aim for, across short lines
+    uint32_t revision_ = 0;
+};
+
+// The file a new note is saved as (Q142): from its first line, "Shopping list!" -> "shopping-list",
+// or "note-<stamp>" when that gives nothing. No extension, no folder.
+std::string nameFromFirstLine(const std::string& firstLine, const std::string& stamp);
+
+// A row's title in the Notes list, from the first bytes of a file: its first line that isn't
+// blank, cut to `maxChars`; "" if there's none.
+std::string titleFrom(const std::string& head, size_t maxChars);
+
+}  // namespace roro::notes