feat(02-02): build core markdown renderer with inline and block elements

- Create src/renderer.rs with public render_markdown(input, width) -> Vec<Line<'static>>
- Implement RenderState with style stack, blockquote depth, list counters, table two-pass collection
- Handle all inline elements: headings H1-H6 (CGA colors + H1/H2 decorators), bold, italic, inline code
- Handle block elements: paragraphs, lists (ordered/unordered/nested with •/◦/▪), blockquotes with │ prefix, horizontal rules, images as [IMAGE: alt] placeholders
- Stub emit_code_block and emit_table delegate to full implementations (Task 2)
- Add mod highlighter, renderer, vault declarations to main.rs (wiring for Plan 03)
- cargo check passes clean (19 dead_code warnings expected — modules unwired until Plan 03)

src/main.rs

@@ -1,7 +1,10 @@
 mod app;
 mod config;
+mod highlighter;
+mod renderer;
 mod signals;
 mod terminal;
+mod vault;
 
 fn main() {
     // ── PRE-TERMINAL PHASE ────────────────────────────────────────────────────