feat(tui): surface compression count in Ink status bar

Parity with the classic CLI status bar (PR #18579). The Python backend
already exposes 'compressions' on SessionUsageResponse; this wires it
through the Ink Usage type and renders 'cmp N' next to the duration
segment of StatusRule.

- types.ts Usage: add optional compressions field
- appChrome.tsx StatusRule: render 'cmp N' when > 0, color-tiered by
  pressure (muted <5, warn 5-9, error 10+)
- Plain text 'cmp' token (no emoji) matches PR #18579's original author
  rationale and avoids Ink layout drift from VS16 emoji width

ui-tui/src/types.ts

@@ -160,6 +160,7 @@ export interface SessionInfo {
 
 export interface Usage {
   calls: number
+  compressions?: number
   context_max?: number
   context_percent?: number
   context_used?: number