feat(examples): add nano-style text editor (SDL2)

examples/SDL2_Editor/.gitignore

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+editor

examples/SDL2_Editor/Makefile

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+CC       ?= gcc
+CFLAGS   ?= -std=c11 -Wall -Wextra -O2
+PKGS     := sdl2 SDL2_ttf
+PKG_CFLAGS := $(shell pkg-config --cflags $(PKGS))
+PKG_LIBS   := $(shell pkg-config --libs $(PKGS))
+
+TARGET := editor
+SRC    := main.c
+
+all: $(TARGET)
+
+$(TARGET): $(SRC)
+	$(CC) $(CFLAGS) $(PKG_CFLAGS) $(SRC) -o $(TARGET) $(PKG_LIBS)
+
+clean:
+	rm -f $(TARGET)
+
+.PHONY: all clean

examples/SDL2_Editor/README.md

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+# SDL2_Editor - nano-style text editor
+
+A real, working text editor for the CardputerZero (320x170), in the spirit of
+GNU nano. Standalone C + SDL2 + SDL2_ttf, no external services.
+
+## Features
+
+- Line-array text buffer with **insert / delete / newline** editing
+- **Vertical and horizontal scrolling** with a line-number gutter
+- Blinking text **cursor**; `L:C` position in the status bar
+- **Selection** (Shift + arrows / Home / End) with on-screen highlight
+- **Cut / copy / paste** through the real system clipboard (SDL clipboard)
+- **File open / save / save-as**, with a modified (`*`) indicator and a
+  save-on-quit prompt
+- nano-style status bar and `Ctrl`-key command set
+
+## Keys
+
+| Key | Action |
+|-----|--------|
+| arrows / PgUp / PgDn | move cursor / scroll |
+| Home / End (or `^A` / `^E`) | start / end of line |
+| Shift + move | extend selection |
+| `^S` | save (prompts for a name if the buffer is unnamed) |
+| `^O` | save-as (edit the name) |
+| `^K` | cut selection (or the whole current line) |
+| `^C` | copy selection |
+| `^U` / `^V` | paste |
+| `^G` | select all |
+| Backspace / Delete | delete |
+| `^Q` / `^X` | quit (asks to save if modified) |
+| ESC | cancel a prompt / clear selection |
+
+## Usage
+
+```
+editor [file]
+```
+
+With no argument it opens an empty buffer. The packaged launcher opens a
+default scratch file (`$HOME/notes.txt`) so `^S` always has somewhere to
+write; pass a path (or set `EDITOR_FILE`) to edit a specific file.
+
+## Build
+
+`make` (needs `libsdl2-dev`, `libsdl2-ttf-dev`; CI installs them via
+`packaging/ci-deps.sh`). Uses the monospace DejaVu font shipped in
+`fonts-dejavu-core`.