v1.1.1: Public error contract hardening

pixtreme 1.1.1 hardens the public error contract. Out-of-contract inputs that previously leaked raw interpreter
exceptions now raise the documented actionable ValueError diagnostics. Public signatures and successful-path
behavior are unchanged.

Fixed

• px.io.to_array() and the eight wire-format exporters now reject non-Frame inputs with actionable ValueError
  diagnostics instead of leaking attribute errors.
• px.io.read_image(), write_image(), read_header(), and read_lut() now translate invalid path types into
  actionable ValueError diagnostics while retaining the underlying cause.
• px.transform.resize() now rejects non-finite or unrepresentable factor-derived dimensions before rounding or
  allocation and guides callers to a smaller factor or explicit dimensions.
• px.composite.merge() now names its actual public path when guiding callers after a non-Frame input.
• Shared finite-real validation now translates numeric conversion failures into the parameter's actionable
  ValueError instead of leaking OverflowError.
• The px.io.to_array() docstring now describes its function input and links to px.values.quantize() correctly.