EXIF auto-orient on load

[swperb]

Why / shareable event: fixes sideways phone photos automatically — a real, everyday annoyance. Ships as "imgcli now respects EXIF orientation."

Scope

• parse the JPEG EXIF Orientation tag on load
• apply the corresponding rotate/flip before filters run
• --no-autorotate to disable
• test against the 8 EXIF orientation values

Notes: stb doesn't do this — parse the tag ourselves (small, self-contained).