fix(tui): give inline_diff segments blank-line breathing room

Visual polish on top of the segment-anchor change: diff blocks were
butting up against the narration around them. Tag diff-only segments
with `kind: 'diff'` (extended on Msg) and give them `marginTop={1}` +
`marginBottom={1}` in MessageLine, matching the spacing we already
use for user messages. Also swaps the regex-based `diffSegmentBody`
check for an explicit `kind === 'diff'` guard so the dedupe path is
clearer.

ui-tui/src/types.ts

@@ -102,7 +102,7 @@ export interface ClarifyReq {
 
 export interface Msg {
   info?: SessionInfo
-  kind?: 'intro' | 'panel' | 'slash' | 'trail'
+  kind?: 'diff' | 'intro' | 'panel' | 'slash' | 'trail'
   panelData?: PanelData
   role: Role
   text: string