feat: issue #32 markdown editor for TUI

specs/tui.allium

@@ -33,12 +33,12 @@ rule SidebarNavigation {
 
 rule OpenEntry {
     when: TuiKeyPressed(code: "enter")
-    -- Posts open in the rendered markdown preview by default (title +
-    -- textarea still seeded from the persisted form; ctrl+e switches to
-    -- editing). New empty posts created with "n" open in the editor
-    -- instead — there is nothing to preview yet. Images open in the
-    -- terminal image preview. Other entities report that terminal
-    -- editing is not available yet.
+    -- Posts open in the editor by default (title + textarea seeded from
+    -- the persisted form; syntax highlighting and word wrap on; ctrl+e
+    -- switches to the rendered preview). New empty posts created with
+    -- "n" open in the editor as well. Images open in the terminal image
+    -- preview. Other entities report that terminal editing is not
+    -- available yet.
     ensures: TuiState.editing_post.updated()
 }
 
@@ -50,17 +50,27 @@ rule SaveAndPublish {
     ensures: PostPersisted()
 }
 
-rule WrappedPreview {
+rule EditorMode {
     when: TuiKeyPressed(code: "e", modifiers: "ctrl")
-    -- ctrl+e toggles between the read-only word-wrapped Markdown
-    -- preview (the default mode when opening a post — rendered through
-    -- tui-markdown so headings/emphasis/lists are styled) and the
-    -- editing textarea (which cannot soft-wrap, an upstream
-    -- ratatui-textarea limitation); keys in preview never edit content,
-    -- and esc returns to the sidebar from either mode.
+    -- ctrl+e toggles between the syntax-highlighted editor (the default
+    -- mode when opening a post) and the read-only rendered-Markdown
+    -- preview (rendered through tui-markdown so headings/emphasis/lists
+    -- are styled); keys in preview never edit content, and esc returns
+    -- to the sidebar from either mode.
     ensures: PreviewToggled()
 }
 
+rule CodeEditorWrapping {
+    when: TuiState.editing_post.updated()
+    -- The post editor renders the buffer with syntax highlighting
+    -- (markdown with [[macro]] accents for posts, HTML+Liquid for
+    -- templates, Lua for scripts), soft-wraps long lines to the
+    -- viewport width, keeps the cursor's visual row visible while
+    -- scrolling, and highlights the cursor's source line. There is no
+    -- line-number gutter.
+    ensures: TuiState.updated()
+}
+
 rule AiQuickAction {
     when: TuiKeyPressed(code: "g", modifiers: "ctrl")
     -- One-shot AI post analysis (title/excerpt suggestions). Gated by