feat(02-01): add syntax highlighter module with CGA color mapping

- Create src/highlighter.rs with OnceLock-based one-time syntect initialization
- Implement init_highlighter(), syntax_set(), theme_set() accessors
- Implement syntect_color_to_cga() with Euclidean RGB-to-CGA 16-color quantization
- Implement highlight_code() returning Vec<Line<'static>> using base16-ocean.dark theme
- LinesWithNewlines iterator preserves newlines for syntect grammar correctness
- Module verified to compile cleanly (tested via temporary mod registration)

src/highlighter.rs

@@ -0,0 +1,171 @@
+use std::sync::OnceLock;
+use syntect::easy::HighlightLines;
+use syntect::highlighting::ThemeSet;
+use syntect::parsing::SyntaxSet;
+use ratatui::text::{Line, Span};
+use ratatui::style::Style;
+
+// ── Module-level OnceLock statics ─────────────────────────────────────────────
+
+static SYNTAX_SET: OnceLock<SyntaxSet> = OnceLock::new();
+static THEME_SET: OnceLock<ThemeSet> = OnceLock::new();
+
+// ── Initialization ────────────────────────────────────────────────────────────
+
+/// Initialize syntect's embedded syntax and theme sets.
+///
+/// Must be called once before any call to `highlight_code()`.
+/// Safe to call multiple times — subsequent calls are no-ops.
+pub fn init_highlighter() {
+    SYNTAX_SET.get_or_init(SyntaxSet::load_defaults_newlines);
+    THEME_SET.get_or_init(ThemeSet::load_defaults);
+}
+
+// ── Accessors ─────────────────────────────────────────────────────────────────
+
+/// Return a reference to the initialized `SyntaxSet`.
+///
+/// # Panics
+///
+/// Panics if `init_highlighter()` has not been called.
+pub fn syntax_set() -> &'static SyntaxSet {
+    SYNTAX_SET.get().expect("init_highlighter() must be called before syntax_set()")
+}
+
+/// Return a reference to the initialized `ThemeSet`.
+///
+/// # Panics
+///
+/// Panics if `init_highlighter()` has not been called.
+pub fn theme_set() -> &'static ThemeSet {
+    THEME_SET.get().expect("init_highlighter() must be called before theme_set()")
+}
+
+// ── CGA color mapping ─────────────────────────────────────────────────────────
+
+/// Map a syntect RGB color to the nearest CGA 16-color ratatui `Color`.
+///
+/// Uses Euclidean distance in RGB space to find the closest match among
+/// the classic 16-color CGA palette.
+pub fn syntect_color_to_cga(c: syntect::highlighting::Color) -> ratatui::style::Color {
+    use ratatui::style::Color;
+
+    // CGA 16-color palette: (r, g, b, ratatui Color variant)
+    const CGA: &[(u8, u8, u8, Color)] = &[
+        (0,   0,   0,   Color::Black),
+        (170, 0,   0,   Color::Red),
+        (0,   170, 0,   Color::Green),
+        (170, 170, 0,   Color::Yellow),
+        (0,   0,   170, Color::Blue),
+        (170, 0,   170, Color::Magenta),
+        (0,   170, 170, Color::Cyan),
+        (170, 170, 170, Color::Gray),
+        (85,  85,  85,  Color::DarkGray),
+        (255, 85,  85,  Color::LightRed),
+        (85,  255, 85,  Color::LightGreen),
+        (255, 255, 85,  Color::LightYellow),
+        (85,  85,  255, Color::LightBlue),
+        (255, 85,  255, Color::LightMagenta),
+        (85,  255, 255, Color::LightCyan),
+        (255, 255, 255, Color::White),
+    ];
+
+    let mut best = Color::White;
+    let mut best_dist = u32::MAX;
+
+    for &(r, g, b, color) in CGA {
+        let dr = (c.r as i32 - r as i32).pow(2) as u32;
+        let dg = (c.g as i32 - g as i32).pow(2) as u32;
+        let db = (c.b as i32 - b as i32).pow(2) as u32;
+        let dist = dr + dg + db;
+        if dist < best_dist {
+            best_dist = dist;
+            best = color;
+        }
+    }
+
+    best
+}
+
+// ── Syntax highlighting ───────────────────────────────────────────────────────
+
+/// Highlight a code string into a list of styled ratatui lines.
+///
+/// Looks up the syntax definition for `lang` (by token first, then by name),
+/// falling back to plain text if no match is found. Applies the
+/// `base16-ocean.dark` theme with colors quantized to the CGA 16-color palette.
+///
+/// Returns `Vec<Line<'static>>` — all content is owned, no lifetime leakage.
+///
+/// # Panics
+///
+/// Panics if `init_highlighter()` has not been called.
+pub fn highlight_code(code: &str, lang: &str) -> Vec<Line<'static>> {
+    let ss = syntax_set();
+    let ts = theme_set();
+
+    let syntax = ss
+        .find_syntax_by_token(lang)
+        .or_else(|| ss.find_syntax_by_name(lang))
+        .unwrap_or_else(|| ss.find_syntax_plain_text());
+
+    let mut highlighter = HighlightLines::new(syntax, &ts.themes["base16-ocean.dark"]);
+
+    let mut result = Vec::new();
+    for line in LinesWithNewlines::new(code) {
+        let ranges = highlighter
+            .highlight_line(line, ss)
+            .unwrap_or_default();
+
+        let spans: Vec<Span<'static>> = ranges
+            .into_iter()
+            .map(|(style, text)| {
+                let fg = syntect_color_to_cga(style.foreground);
+                Span::styled(text.to_string(), Style::default().fg(fg))
+            })
+            .collect();
+
+        result.push(Line::from(spans));
+    }
+
+    result
+}
+
+// ── LinesWithNewlines iterator ────────────────────────────────────────────────
+
+/// Iterator that yields lines with their trailing newline preserved.
+///
+/// syntect's `highlight_line()` expects lines to include the `\n` terminator
+/// so that language grammars can match end-of-line anchors correctly.
+struct LinesWithNewlines<'a> {
+    remaining: &'a str,
+}
+
+impl<'a> LinesWithNewlines<'a> {
+    fn new(s: &'a str) -> Self {
+        Self { remaining: s }
+    }
+}
+
+impl<'a> Iterator for LinesWithNewlines<'a> {
+    type Item = &'a str;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.remaining.is_empty() {
+            return None;
+        }
+        match self.remaining.find('\n') {
+            Some(pos) => {
+                let line = &self.remaining[..=pos]; // include the \n
+                self.remaining = &self.remaining[pos + 1..];
+                Some(line)
+            }
+            None => {
+                // Last line without trailing newline
+                let line = self.remaining;
+                self.remaining = "";
+                Some(line)
+            }
+        }
+    }
+}