Gate preview mesh updates to value-input descendant changes

[tracygardner]

Motivation

• Prevent runtime/statement-chain changes (highlights, tints, DO/next chains) from driving live preview mesh updates by only trusting configuration/value-input subtrees.
• Consolidate and reuse the value-input ancestry check to remove ad-hoc/special-case traversal logic (e.g. create_map MATERIAL) and reduce duplication.
• Make parent-propagation logic in block change handling more precise so only provably relevant child changes update meshes.

Description

• Add isValueInputDescendantOf(containerBlock, changedBlock) in blocks/blocks.js that returns true for self-updates or when every parent hop from changedBlock to containerBlock is through a VALUE input connection.
• Replace scattered ad-hoc checks in handleFieldOrChildChange with the new helper while keeping direct owner-field updates intact.
• Gate parent-propagation in handleBlockChange so updateOrCreateMeshFromBlock is only called when a resolvable changedBlock exists and isValueInputDescendantOf(block, changedBlock) is true, and add the inline comment: "Only configuration inputs (value-input subtree) affect preview mesh; runtime statement blocks do not."
• load_object and other load/create blocks that delegate to handleBlockChange now automatically benefit from the stricter VALUE-input-descendant gating without per-block duplication.

Testing

• Ran npx eslint blocks/blocks.js blocks/models.js, which reported existing repository-level lint errors unrelated to the new helper and change (the lint run failed).
• No additional automated unit or integration tests were executed as part of this change.

---

Codex Task