Caster's Compendium v0.1.6-hotfix.1

Caster's Compendium v0.1.6-hotfix.1

This prerelease hotfix corrects the selectable backgrounds introduced in
v0.1.6.

Background rendering fix

• Return the background stack to Fyne as a concrete container that its renderer
  can traverse.
• Render the selected artwork, readability layer, and application controls in
  their intended order.
• Redraw the main menu immediately after a background setting is saved.
• Add a regression test for the concrete renderer-compatible container type.

The original v0.1.6 implementation used a custom type embedding a Fyne
container. Although it satisfied Go's canvas-object interface, Fyne only
traverses children for concrete containers or widgets, leaving every nested
layer unpainted. This affected all platforms and was first observed in the
Debian package.

AI transparency

Hybrid identified and reported the packaged rendering failure and authorized
OpenAI Codex to implement and publish this focused hotfix. See
AI_STATEMENT.md for the complete contribution
disclosure.

Release verification

The hotfix includes SHA-256 checksums, an SPDX software bill of materials, and
GitHub artifact attestations. Windows and macOS packages remain unsigned and
may display a first-launch security prompt.