v0.8.0: Markdown + Code + Diff rendering module

Add cl-tui.markdown package with:
- Markdown parser: headings, paragraphs, bold, italic, inline-code, links,
  code blocks, blockquotes, lists, thematic breaks
- Syntax highlighting: Lisp, Python, JavaScript, Bash with keyword,
  builtin, comment, number, function coloring
- Diff renderer: colorized unified diff (+/-/@ lines)
- Terminal renderer: ANSI escape sequences via backend-style functions
- 67 tests, 100% passing
- All parser helpers use values returns (not cons) for multiple-value-bind

ASDF: v0.7.0 -> v0.8.0, new markdown module + test suite

src/components/markdown-package.lisp

@@ -0,0 +1,65 @@
+;;; markdown-package.lisp — Package definition for cl-tui.markdown
+
+(defpackage :cl-tui.markdown
+  (:use :cl :fiveam)
+  (:export
+   ;; Data structures
+   #:make-md-node
+   #:md-node-p
+   #:md-node-text
+   ;; Parsing
+   #:parse-blocks
+   #:parse-inline
+   ;; Highlighting
+   #:*syntax-highlighters*
+   #:highlight-code
+   ;; Diff
+   #:classify-diff-line
+   ;; Rendering
+   #:render-md
+   #:render-md-node
+   #:render-markdown
+   #:render-inline
+   ;; Styles
+   #:apply-style
+   #:apply-styles
+   ;; Tests (exported test symbols for ASDF integration)
+   #:heading-parsing
+   #:heading-levels
+   #:heading-with-inline-formatting
+   #:paragraph-parsing
+   #:paragraph-multi-line
+   #:bold-parsing
+   #:italic-parsing
+   #:bold-italic-combined
+   #:inline-code-parsing
+   #:link-parsing
+   #:code-block-parsing
+   #:code-block-unknown-language
+   #:blockquote-parsing
+   #:list-item-parsing
+   #:ordered-list-parsing
+   #:thematic-break-parsing
+   #:highlight-lisp-keyword
+   #:highlight-lisp-builtin
+   #:highlight-unknown-language
+   #:highlight-comment
+   #:classify-diff-added
+   #:classify-diff-removed
+   #:classify-diff-hunk
+   #:classify-diff-context
+   #:render-heading-output
+   #:render-paragraph-output
+   #:render-thematic-break-output
+   #:render-code-block-output
+   #:render-diff-block-output
+   #:markdown-integration
+   #:render-markdown-string
+   ;; Internal (for testability)
+   #:classify-line
+   #:split-string-into-lines
+   #:find-closing-marker
+   #:string-prefix-p
+   #:tokenize-line
+   #:apply-highlight-token
+   #:apply-highlight-style))