Function mountStatusBar

Mounts the status bar into container and returns a StatusBarHandle for external updates and cleanup.

Segments rendered (left to right):

1. Size – static total file size.
2. Offset – current cursor position in decimal and hex.
3. Byte – value at the cursor as hex (dec) 'char'.
4. Binary – 8-bit binary representation of the current byte.
5. Selection – byte count and hex preview of the active selection.
6. Pos – percentage progress bar.