Adds a trust bar for size distribution result plots

[jellybean2004]

Description

Fixes #3271

This PR adds a Trust Bar to the Size Distribution perspective, inspired by IRENA-style plots. The bar visually indicates which diameter region is reliable based on the fitted Q-range, using a smooth red/yellow/green gradient.

Key changes

• Adds a horizontal trust bar above the size distribution plot.

• Uses two physical trust boundaries derived from the fit range as defined in SizeDistributionLogic:

  • D_low = 1.8 * pi / Qmax
  • D_high = 0.95 * pi / Qmin

• Displays:

  • red for out-of-range / unreliable diameters
  • yellow for transition regions
  • green for the trusted region

• A small helper class (TrustBar) handles:

  • drawing the gradient bar
  • clearing it
  • updating it when the x-axis limits change

• The bar stays aligned when the plot is zoomed or rescaled.

How Has This Been Tested?

Manually tested functionality.

Review Checklist:

Documentation

• There is nothing that needs documenting
• Documentation changes are in this PR
• There is an issue open for the documentation Add docs for trust bar in Size Distribution #4016

Installers

• There is a chance this will affect the installers, if so
  • Windows installer (GH artifact) has been tested (installed and worked)
  • MacOSX installer (GH artifact) has been tested (installed and worked)
  • Wheels installer (GH artifact) has been tested (installed and worked)

Licensing

• The introduced changes comply with SasView license (BSD 3-Clause)