[Arjan-Woltjer]

Companion to Open-Agriculture/AgIsoStack-rs#48; fixes #35.

Depends on the AgIsoStack-rs change landing on daan/terminal-designer-changes first — until then this does not build against the pinned dependency, which is why it is a draft. Cargo.toml is deliberately left pointing at upstream.

What

AgIsoStack-rs is making line_attributes (Output Line / Rectangle / Ellipse / Polygon), font_attributes (Input/Output String/Number) and Input Boolean foreground_colour NullableObjectId, because ISO 11783-6 allows NULL there and real pools (John Deere) use it. This adapts the Designer, following exactly how it already handles fill_attributes:

• Configuring: the selectors for those fields gain a "None" entry and show "None" when NULL; the "(view)" link only appears when a reference is set.
• Rendering:
  • NULL line attributes: an Output Line draws nothing; a Rectangle or Polygon keeps its fill and gets no outline.
  • NULL font attributes: text renders with the Designer's default font attributes instead of a red "Missing FontAttributes" error — NULL is legal, a dangling reference still shows the error.
  • NULL foreground colour on an Input Boolean: falls back to black, as a missing one already did.
• Defaults: new objects start with these references at NULL rather than pointing at object 0, which normally does not exist.
• default_font_attributes is made pub for the rendering fallback.

Verification

Built locally against the AgIsoStack-rs branch: cargo check --lib, cargo test --lib and cargo fmt --check clean; cargo check --lib warnings are unchanged from the base branch. Loading the John Deere StarFire pool then gives all 9 619 objects and its Working Set; the 31-byte reproducer from the issue gives 3 objects and a Working Set. Not run: the wasm32 check (no platform-specific code touched).

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🔵 Needs a closer look

The changes span configuration, defaults, and rendering behavior and warrant human review.

Pull request overview

Updates the Designer to support legal NULL line, font, and foreground attribute references.

Changes:

• Adds NULL-aware selectors and conditional view links.
• Implements rendering fallbacks for nullable references.
• Initializes new references as NULL and exposes default font attributes.

File summaries

File	Description
src/object_rendering.rs	Handles nullable rendering references.
src/object_defaults.rs	Initializes affected references as NULL.
src/object_configuring.rs	Adds “None” options and conditional view links.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 0
• Review effort level: Lite

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