feat: 顶层 API + PNG/SVG/ASCII 渲染器

core/src/render/ascii.rs

@@ -1 +1,27 @@
-// FIXME: ASCII 渲染器 — Task 10
+use crate::qr::QrCode;
+
+/// 终端 ASCII 渲染：██ 表示暗模块，"  " 表示亮模块
+pub fn render_ascii(qr: &QrCode, invert: bool) -> String {
+    let size = qr.size() as usize;
+    let margin = qr.margin as usize;
+    let total = size + 2 * margin;
+
+    let dark_char = if invert { "  " } else { "██" };
+    let light_char = if invert { "██" } else { "  " };
+
+    let mut result = String::new();
+    for y in 0..total {
+        for x in 0..total {
+            let mx = x.saturating_sub(margin);
+            let my = y.saturating_sub(margin);
+            let is_dark = if mx < size && my < size {
+                qr.modules()[my][mx]
+            } else {
+                false
+            };
+            result.push_str(if is_dark { dark_char } else { light_char });
+        }
+        result.push('\n');
+    }
+    result
+}