refactor: extract pkg/charset and internal/event packages

Extract ConvertChunk/FormatHexFrame into pkg/charset (zero external deps).
Extract UIEvent/UIEventKind/UIPanelKind types into internal/event.
Update all references across main package to use qualified imports.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/event/event.go

@@ -0,0 +1,30 @@
+// Package event defines UI event types shared between app, console, and tui packages.
+package event
+
+// UIEventKind classifies a UI event.
+type UIEventKind int
+
+const (
+	UIEventOutput UIEventKind = iota
+	UIEventStatus
+	UIEventModal
+	UIEventPanel
+)
+
+// UIPanelKind identifies a modal panel type.
+type UIPanelKind int
+
+const (
+	UIPanelNone UIPanelKind = iota
+	UIPanelForward
+	UIPanelPlugin
+	UIPanelMode
+)
+
+// UIEvent is emitted by the app core and consumed by TUI or console frontends.
+type UIEvent struct {
+	Kind  UIEventKind
+	Title string
+	Text  string
+	Panel UIPanelKind
+}